Support for "literate" CoffeeScript (.litcoffee)

This adds support for the new ‘literate’ CoffeeScript mode [announced this morning](http://coffeescript.org/#literate) as part of CoffeeScript 1.5.

Changes:
* bumped `coffee-script` dependency to version 1.5
* modified coffee.js task to enable `literal` option, depending on file extension
* added new test case

To use ‘literate’ mode, you just need to use a `.litcoffee` file extension instead of `.coffee`.