sortable has_many

This is a revival of #2614, #881, #2117, #880
Its a still a work in progress, but I wanted to post this early on to make sure I'm on the right path and this doesn't slip through the cracks.

TODO:
- ~~force initial ordering~~
- ~~add an indicator icon to show that the fieldsets can be dragged~~
  - http://activeadmin-icons.herokuapp.com/admin
  - http://rdoc.info/github/gregbell/active_admin/master/ActiveAdmin/Iconic
  - https://groups.google.com/forum/#!topic/activeadmin/SpdnDLf4i5I
  - #2621
- ~~add css to change the cursor to indicate that the fieldset is draggable~~
- add tests
  - http://www.dixis.com/?p=626
  - http://forum.jquery.com/topic/fire-events-to-simulate-drag-n-drop
- ~~add docs~~
- ~~deal with nested sortable has_many fieldsets~~