added js dependency support for rails

I added this for the case when you want to use some functions in your helpers that are defined in other js files.
It can be used like so:

    ShtRails.configure do |config|
      # ...
      config.dependencies = [
          ShtRails::ContentDependency.new('alert("here");'); # inline javascript content
          ShtRails::AssetDependency.new('I18n'), # gem asset
          ShtRails::AssetDependency.new('vendor/underscore') # app/assets/javascripts/vendor/underscore.js
      ]
    end