Remove deprecated grunt.util.async / Update devDependencies

[shinnn]

• I replaced grunt.util.async.forEach with each-async. grunt.util.async is deprecated.
• I updated grunt-contrib-jshint and grunt-contrib-nodeunit.
• I removed proxyquire from dependencies. This project doesn't use it.
• I updated the file array of jshint task. This repository doesn't include lib directory and test directory.
• I updated .jshintrc.
  • I removed the global variables of Mocha because this project doesn't use it.
  • es5 option is now set per default (http://www.jshint.com/blog/2013-05-07/2-0-0/).
  • From JSHint v2.5.0, trailing and smarttabs options are removed.
• I updated the image format of the build status badge. SVG badges look beautiful on retina displays.

[passy]

Wow, sorry for being so embarrassingly late. Most of the commits were still relevant. I cherry-picked everything on top of master. Thanks especially for formatting the commit messages correctly! <3

[shinnn]

No problem! I'm glad I could help.