Activity objects in Inbox

Instead of adding `Note`,`Comment` objects directly in the `Inbox`, we use `Activity` objects to describe an activity and its components (Activity.verb, Activity.actor, Activity.obj, Activity.generator, and so on..)


Current implementation works with `Inbox`.
It should be usable as it is now, but there's still some work to do..
I have removed the `actor` attribute from the `Note` and `Comment` classes as it is now stored in `Activity` and i think we can remove some more code from them like `summary`, `deleted`, and `if "object" in data` statements but i wanted to make we agree on the basic idea to do this first. :)