Added dynamite support for deleting from sets.

Hello @nicks, 

Please review the following commits I made in branch 'jfuchs-delete-from-attribute'.

e5e53900a4ce90e8c3b125cdf8da7135b02c1000 (2013-09-03 14:10:12 -0700)
Added dynamite support for deleting from sets.


R=@nicks