Make django-taggit-live more reuseable

This pull-request contains some small changes to make django-taggit-live more reuseable:

* make it work if settings.STATIC_URL is not "/static/"
* make if work if 'taggit.urls' are not included at the root of the url-space
* make Django-Admin integration easier by falling back to django.jQuery is, jQuery is not available
* Fix a Django 1.7 compatiblity problem. Django 1.7 no longer ships with simplejson.