Many Under the hood changes

This PR includes many Under the hood changes, like (not in a particular order):
* __Important__ XSS prevention (unfortunately, we don't have anything better than `$("<div/>").text(var).html()` right now, but that's gonna do it)
* Remove the Starter Issues/Features dummy content (just like we did with the New Features content before) and show a spinner instead
* Remove those goddamn `.DS_Store` files and add a `.gitignore` to exclude them
* Use more jQuery style (makes everything easier and looks nicer) and Deferreds + handlers
* Adapt the Brackets code style/coding conventions, at least a little (mostly about indentation, spaces and `"` vs `'`)
* _GitHub API_ (starter issues): Tell the API to only deliver 6 bugs at max, so that we don't need to handle slicing it ourselves
* Retrieve the count of languages Brackets has been translated into and put into the Contribute page's section about translating
* Use some ECMAScript5 features like `Array.forEach` and `Array.some`. That [won't work in old IEs](http://kangax.github.io/compat-table/es5/), but that's not an issue according to https://github.com/adobe/brackets.io/pull/67#issuecomment-42252995
* Use `RegExp.test()` for OS testing
* Remove non-necessary duplicate #blogposts container

If you don't like any piece listed here, ping me
If you don't know what a particular piece of code is for, ask me

Thanks.

Demo: https://dc7296e2001a9014f1d00ea89b966216336fe5b7.googledrive.com/host/0B986Uw1nLGY7XzNlY2hXVXI5X1U/index.html