Switch from npm to Yarn

Since Yarn (1) is fully compatible with npm (2), but provides way more
useful features and uses a stable dependency solution strategy,
snowsaw now uses Yarn by default. Therefore the npm specific
`package-lock.json` file has been removed and replaced with Yarn's
`yarn.lock` file.

References:
  (1) https://yarnpkg.com
  (2) https://www.npmjs.com

Epic GH-33
Resolves GH-81

Author: arcticicestudio

.gitignore

@@ -19,12 +19,16 @@
 # +------+
 **/*.log
 logs/
+.npm/
 npm-debug.log*
+npm-error.log*
+yarn-debug.log*
+yarn-error.log*
+.yarn-integrity
 
 # +---------+
 # + Node.js +
 # +---------+
-.npm/
 **/node_modules/
 
 # +-------------------+