Added the ability to set a custom history_date.
I fixed the example for a custom history_date, as it was refering to
my_poll.history_date instead of my_poll._history_date.
I also rewrote the documentation to make it better understandable.
Fixed import of datetime.now as noticed by treyhunner.
Merge branch 'set-custom-history_date'

Conflicts:
	CHANGES.rst
	docs/advanced.rst
	simple_history/models.py
	simple_history/tests/tests.py