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.
Make Stripe optional

While Stripe is handy for the paid service, it might not be as useful when
hosting your own instance (or when doing local development for that matter).
This is an attempt to make the Stripe integration optional by disabling it if
the Stripe keys are missing.