Wrap Dajax object in a closure and attach it to the window object.

This fixes a visibility issue which manifests itself if you have a minification
process which wraps all the files inside a closure (e.g. function(){}()). An
example of such process is using django-pipeline.
Merge branch 'fix_jquery_dajax_scoping_issue' of github.com:Kami/django-dajax
fixes jorgebastida/django-dajax#66