Updated .travis.yml.
Fix issue #115

Moved "bailout" code to the top of the Filter.filter method, so it catches empty list and tuple.
Ammended my fix for #115

Instead of moving bailout code, added condition on list length, to prevent unpacking of empty list.
Fixed test models for python3.x
Added exclusion filters support
Address passing lookup type to tilter method.

LookupTypeField.compress() now returns Lookup (a simple namedtuple) instance.
Filter.filter() now checks for value being a Lookup instance and handles setting lookup_type
Adjust tests to handle these changes.

Fixes: #115
Fix typo in LookupTypeField.compress.
Updated docs for generic view from "filter_class" to "filterset_class" as expected.
Fix some formatting in docs.

The indentation was causing the whole block to render as a `<pre>`.
Add warning about lookup type for CharFields/TextFields

I had an issue with it and #django IRC said that it's quite common issue so... here we go!
Fix line-lenght
Fix typo in widgets docs
docs: document filter_overrides

Found it while i was going to reimplement it :)
Fixed warnings in the testsuite.
Add Python version classifiers

Fixes gh-147.