Implement --all, refactor code, add tests

[kfranqueiro]

This adds support for --all, --platform=all, and --arch=all.

In order to accommodate outputting multiple directories for multiple platforms and architectures, this also implements a new directory structure under the output folder (distinguished by both platform and
arch). This structure is applied even to OS X distributions, which formerly were output directly to an .app folder. This could be considered a backwards-incompatible change.

One other backwards-incompatible change is the value that the packager function passes to the callback, which is now always an array of paths, rather than just a single path.

The behavior of the icon option has also been modified to use its basename and apply .ico or .icns depending on platform, to make it usable with --all and --platform=all. This attempts to maximize backwards compatibility, by allowing a full filename to be specified, but replacing the filename's extension with what is appropriate for each target platform. Alternatively, the extension can now be omitted.

In the process of implementing this, it became evident that some things were being done in 3 different places, and weren't always being done consistently, so I've deduplicated everything I could.

This also includes a few other changes to improve stability for multi-target runs, and other fixes:

• Avoid targeting darwin if the environment doesn't support symlinks,
  to avoid the process bailing out on Windows
• Implement --overwrite centrally in index.js such that it explicitly
  skips if an output directory already exists, for consistency with
  all target platforms and to avoid any possible errors that would halt
  operation during one target of a multi-target run
• Use ncp instead of mv to move to finalPath, which avoids flakiness
  I noticed when testing on Windows 8 especially with multi-target runs
• Simplify temp directory logic by using a nested structure, so there
  is only one top-level directory to clean up
• Reinstate fix from get rid of source dir in path for ignore filter matching #55 which seems to have been clobbered by a later
  merge
• linux.createApp now resolves to the final output directory;
  it was formerly resolving to the executable path
• mac.createApp now replaces space with underscore in bundle IDs
• Only the platform modules that are needed are loaded
• The win32 module only loads rcedit if needed

This also fixes a couple of missing updates to docs (readme/usage).

The added tests cover the following cases:

• Basic runs for a specific version/platform/arch with default options
  (testing for existence of expected output files/folders)
• Runs for each target platform with out set
• Runs for each target platform with asar set
• Runs for each target platform with prune set
• Runs for each target platform with ignore set
  • This includes testing ignore including a path separator
  • This also includes testing that ignore entries are applied
    with the app folder trimmed
• Runs for each target platform with overwrite set / not set
• Runs with all set, with arch or platform each set to all,
  and with platform set to multiple platforms (comma-delimited or
  array)
• Tests specifically for the darwin target platform, to test
  icon and sign

The test logic is organized such that the version of Electron being tested can be configured once centrally (in config.json), and the tests will not run until after downloading any necessary electron versions (to avoid timing out due to long downloads).

This addresses the following issues:

• Resolves --all option #40
• Resolves --out inconsistency #38
• Resolves Issue with outputting Win32 versu Win64 apps #70
• Works around Building Mac applications on Windows generates no output #71
• Resolves Should ignore comparisons trim opts.dir first? #84 by reinstating get rid of source dir in path for ignore filter matching #55
• Resolves we need some tests! #77 with tests

[kfranqueiro]

I've run the tests on OS X 10.10, Xubuntu, Windows 7, and Windows 8, and verified they passed before submitting this PR.

[max-mapper]

EPIC

[max-mapper]

published as 5.0.0 and added @kfranqueiro as a collaborator. thanks a ton for this!

[kfranqueiro]

Awesome! Thanks for accepting it. Glad my OCD paid off :)