Adding priorities to view_action's

I've added a `priority` kwarg to the view_action decorator and callable (in `models.py`). I also amended the `__setitem__` method to insert keys into the `self._order` keylist at the index specified by the priority; if no priority is given the key is just appended to `self._order`.

View actions that register themselves with clashing priorities simply use Python's `insert()` function behavior (last in pushes the first-in element back in its order), there's no fancy conflict resolution.

I know some people are persnickety about upping version numbers on packages but I consider this to be an API change even though it won't break backwards compatibility. I can revert that specific change if you don't agree with it.