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
Ignore .env files
Adds test coverage reporting
Added the ability to remove a user from an organization.
Added a test for removing a user from an organization.
Fixed the test failure.
Cleans up W293 warning
Merges pull request for user removal method

Closes gh-46
Updates authors [ci skip]
Merges bugfix version from master
Updates contribution guidelines [ci skip]
Update README.rst

Explain more benefits of current versions and add SVG icons
Use Django 1.7 package for testing
Use get_queryset method by default

Adds handler for get_query_set where the former method is not available
in the base manager class.

Closes gh-48
Move migrations to south_migrations

Django 1.7 will use the `migrations` folder for Django core migrations;
as of South 1.0, South will check first in `south_migrations` for South
generated migrations.
Version bump
Merge branch 'dev' for 0.4.2 release
Update installation documentation