let's make it a bit more Python 3 compatible

Conflicts:
	djcelery/tests/test_schedulers.py
fix PeriodicTaskAdmin (__unicode__)
depend less on 3rd parties to be Py2/3 compatible

Conflicts:
	djcelery/tests/test_schedulers.py
remove unnecessary __unicode__ method from PeriodicTaskAdmin
Adds Piotr Bulinski to AUTHORS