updated Field inheritance pull (fixed)

I updated the code from shelldweller's pull request #40 and merged it into my branch from the trunk (1st commit).

I also added a new test (with supporting model/fields) to verify that custom fields which inherit from models.Field are not filtered by default, but are when included by setting filter_overrides on the FilterSet.  I also included a fix for py3k not allowing non-string kwargs when initializing a dict - as when setting `filter_overrides` (2nd and 3rd commits).  Travis-ci now builds correctly.