Improved & sortable has_many

Hi,

This is is my take on improving the has_many helper:
- removes inline js from the html and moves into a a standalone file (jquery.aa-has-many.js.coffee)
- fixes the bug of newly added formsets appearing outside li.input
- triggers event to allow initialization/destruction of js widgets in the nested form (#1767)
- adds drag & drop sorting (#881, #2117, #880)

This differs from #2117 in the following  respects
- its fully funcational
- it encapsulates all the has_many logic in a class
- the full row is draggable (doesn't use jquery-ui's handle)
- its styled
- it hasn't added any tests

I tried to add tests, but I couldn't get any of the jasmine tests to run, if you can offer some advice, I will gladly add them.