Adding pin payments pin.net.au support
Added Pin Payments models and forms
Merge remote-tracking branch 'upstream/master'

Conflicts:
	billing/gateways/pin_gateway.py
	billing/models/__init__.py
	billing/tests/pin_tests.py
Added support for specifying of which gateway models/admin to activate. Refactored import routines with some magic.
merged #84 - pin payments update