Verb sorting / filtering

Hi once more (don't worry, that's my last PR for today!),

These commits add the ability to filter the actions by verb. They basically add a `VerbsField` to the `Follow` class so that the follow functions / relevant managers methods can filter the actions to return by verb. Useful if you only want to subscribe to given actions relative to an object and not be drown under dozens of irrelevant actions.

Feel free to comment / make suggestions.

Btw, I already have a branch merging the 3 PRs I just made ([unread actions](/justquick/django-activity-stream/pull/186), [deleted items](/justquick/django-activity-stream/pull/187) and this one), it can save some time if you wish to keep all 3 of them (the merge is a bit tedious). It's just a matter of pushing it on my repo.

Thomas