Changing the way of alchemy menubar rendering.

From now we let rails render a partial for the menubar, removed the helper ´alchemy_menu_bar´ and do not build the bar with javascript anymore.

Old usage:  <%= alchemy_menu_bar %>
New usage: <%= render "alchemy/menubar" %>
Merge pull request #444 from robinboening/menubar

Changing the way of alchemy menubar rendering.