Support for HTML5 data elements in content_for

I've added support for HTML5 data elements in content_for

For example:

<% content_tag_for(:li, @person, :data => [:first_name, :last_name]) %>...

would produce the following HTML (assuming @person has the methods first_name and last_name):

<li id="person_123" data-first_name="David" data-last_name="Heinemeier Hansson">...

I've attached a patches for master and 2-3-stable