Merge pull request #45 from mottosso/master

Refactoring and minor fixes
Merge pull request #51 from mottosso/master

Implementation works, but some tests fail. Tests are being revisited in a future commit shortly.
Attempt at #49

All plugins now inherit from the same Filter base class.
There is a convenience function for plugins, to get the relevant
instances called with `self.instances`.
Making Filter.instances a property
Removing accidental plugin
Moving families property to relevant subclasses
Merge pull request #53 from tokejepsen/master

Merging. Good work!
Merge branch 'master' of github.com:abstractfactory/publish