Feature/accept blank incoming msgs

Sending a blank message to the Vumi backend causes RapidSMS to respond to Vumi with HTTP status code 410 and also raises 3 exceptions. A live RapidSMS installation is likely to receive an occasional blank SMS and should be able to handle it in its own way, rather than being forced to raise an exception.

This PR includes changes to the tox suite and other associated changes due to the fact that Django now has a branch for the upcoming 1.6 release. I've therefore added 1.6.X to the tox matrix. Django's master branch will become 1.7 which is the first version that will drop support for Python 2.6.

* Django 1.7 deprecates simplejson in favor of Python's json module
* nose, django-nose and django-tables2 upgraded to fix incompatibilities with Django 1.7