gregbell/active_admin — pull request #1766 by awinabi (2012-11-13 18:25:37+00:00)
Title: #909 - multiple columns to sortable

Adding in this feature for #909.
This pull request will let pass multiple columns to sortable in the following formats

:sortable => [:first_name, :last_name]
:sortable => ["user.first_name", "user.last_name"]
:sortable => "first_name, last_name"

Added in a couple of specs as well