tests for broken related objects with custom Application

See issue #90
Make Application model truly 'swappable'

This also introduces a new non-namespaced setting
OAUTH2_PROVIDER_APPLICATION_MODEL.
merged into master
updated docs for custom Application model setting
link tutorial3 from tutorial2 fixes #99
added docs testing to tox
fixed django versions in tox
Update tutorial_01.rst
Update tutorial_01.rst
Fix a typo in tutorial_01.rst
added requirement file for testing
run converage in tox
make travis run tox fixes #72
run coverage in append mode
merged joepeg:tutorial1-edits