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