Adds base property to get the related name

The related name of an organization to the user model of choice will
differ based on the organization class. This property provides a simple
way of returning that related name.
Adds backend support for different org related names

The invitation backend checks against the provided model - with a
default value if not provided - to identify the related name for
accessing the organizations associated with a given user.

Closes gh-43
Added tree support for Organization model
Added OrganizationTree to admin and added django-admin-mptt dependency