Change IPAddressField to GenericIPAddressField

When using a PostgreSQL database and the client does not pass an IP address you get an inet error.  This is a known problem with PostgreSQL and the IPAddressField.

https://code.djangoproject.com/ticket/5622

It can be fixed by using a GenericIPAddressField instead.
Merge pull request #90 from polvoblanco/master

Change IPAddressField to GenericIPAddressField