Support for "literate" CoffeeScript (.litcoffee)

This adds support for the new ‘literate’ CoffeeScript dialect [announced this morning](http://coffeescript.org/#literate) as part of CoffeeScript 1.5.

Changes:
* upgraded to CoffeeScript 1.5
* modified coffee.js enable 'literal' option depending on file extension
* added new test case

To use literate mode, you simply use a `.litcoffee` file extension instead of `.coffee`. (This is how the `coffee` command works, so I figured this grunt task should work the same way.)

This is my first PR; let me know if there are any problems and I'll try to fix!