Add basic jshint support

This should at least partially address (https://github.com/xolvio/rtd/issues/66). I wasn't quite sure why thresholds were mentioned in the ticket's comments since I view JsHint as more of a binary yes / no versus something that needs thresholds. 

- Added an option to run jshint against app and test code with (potentially different) jshint options specified in rtd.conf.js.
- I thought this would be more work, but there is already a grunt jshint plugin (grunt-contrib-jshint)
- I thought it made sense to exclude the app/.meteor and test/rtd directories since they don't contain application code

Let me know what you think.

Thanks,
Pete