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 'fix_ordering' into get_order_by

* fix_ordering:
  Fix regression in order_by
  explain the "strict" FilterSet option in the docs...created subsection to make it clearer it isn't a Meta option
  keep example FilterSet class name consistent
  fix a few tests due to defaulting strict = True
  - need to differentiate between actual 'None' value and a ValidationError - grab order_by value in for loop, avoids double lookup and eliminates need for get_value function
  change default value for strict to True, and hopefully made the code for filtering with invalid values clearer (also removed _SENTINEL)
  Fixed tests for Django 1.6/1.7.
  Disable python 2.6 on tests against Django master.
  Fixed support for newer wheel versions.
  change FilterSet to invoke form errors and skip filtering on invalid data, returning empty queryset with strict mode

Conflicts:
	django_filters/filterset.py
Merge pull request #106 from timheap/develop

Add `FilterSet.get_order()` method for very custom ordering