vendor directory support

This release is a major step forward for godog, now we do not copy or rewrite any source files. Instead we use standard go tools:

• go test -c -work -o out.test to compile a tested package with test files included. Since it uses standard go command it supports all it's features. We can later maybe look how to add code coverage. It builds it even with CGO support.
• go tool compile - to compile our testmain entry point to run tests.
• go tool link - to create an executable godog suite file

1. We now fully support vendor directory and multiple GOPATH paths.
2. Fixed a bug with --stop-on-failure flag.
3. Added --no-colors flag, to strip ANSI color from output.
4. Now any context func needs to be exported. Same as go test requirement. It will show nice message on this issue.
5. Only go 1.5 and later versions from now on.