Add base activity stream item
Add stream items for discussions and announcements
add stream item for conversations
add activity stream item for system messages
add activity stream items for collections and conferences
Add activity stream item for submission
Add networking call for activity stream.

This involved a slight refactoring of paged items to
accept a transformer instead of just a class. This is ok
because at some point we were going to want the power to
create different items not all of the same class from one
API endpoint.
Add tests for activity stream transformer
Merge pull request #16 from jarsen/master

Activity Stream