Add `FilterSet.get_order()` method for very custom ordering
Fix a typo in the get_order docs

Ordering people by name should be last name, first name; not last name,
last name.
Rename FilterSet.get_order to get_order_by

As requested in #106
Update docs to reflect renamed get_order_by method
Merge branch 'develop' of https://github.com/timheap/django-filter into develop-2

Conflicts:
	tests/test_filterset.py