Literate CoffeeScript support

I rebased pull request #173 on the latest master branch to enable Literate CoffeeScript in Tilt 2.x.

## Using Literate CoffeeScript in a Rails app
### Configure Sprockets
You'll still have to tell Sprockets what to do with `.litcoffee` files [as described by Skalee](https://github.com/rtomayko/tilt/pull/173#issuecomment-14238077); a Sprockets developer stated in sstephenson/sprockets#420 that `.litcoffee` support won't be enabled by default.

### Update Gemfile
For now, you also have to update your Gemfile to use the `mr-vinn/tilt` repo.  However, Tilt is locked to 1.4.1 in the latest stable Rails version (4.1.5), so I applied these commits to Tilt 1.4.1 in a different branch.  Add this to your Gemfile to get a litcoffee-enabled Tilt 1.4.1:

    group :assets do
      gem 'tilt', github: 'mr-vinn/tilt', branch: '1.4.1-litcoffee'
    end