Added actions API for Notifications

We added the missing "Actions" API to the Notification object, but we were kind of in a rush. This can be used for further development though. Needed changes are:
- More tests
- Add more precondition checks

Implemented API:
http://docs.urbanairship.com/reference/api/v3/actions.html

Addresses issue #17 
As can be seen in PushPayload, we generally like to break the build up into two steps - first checking optional variables via preconditions and then returning the already checked variables.