Add JavaScript support

https://trello.com/c/LQsUqhYU/179-support-javascript

* Use JSHint to review JavaScript code
* Allow users to configure JavaScript style guide
* Use jshintrb gem to easily use JSHint in Ruby
Fix style guides link
Specify indentation preference for JavaScript
Merge pull request #405 from soulcutter/patch-1

Update style guide path.
Add dotenv-rails to Gemfile