Rename unread_messages_count to new_messages_count
Convert Inbox to use dict-API
Convert Inbox to dict api, tweak Inboxes.__setitem__

- Inherit from OOBTree
- Convert functions
- Add additional sanity checks
- use zope.interface.verify.verifyObject in __setitem__

Inboxes:
- use zope.interface.verify.verifyObject in __setitem__
dict-API for Conversation, fix new messages accounting

Conversation:
- inherit from LOBTree (long int:object-mappign)
- split key genration out of add_message()
- add update_messages_count() to do the update on conversations
  and inboxes
- implement __setitem__ and __delitem__ for checks and update the
  new messages count.

Inbox:
- forbid to override an existing Conversation in __setitem__ for security
  reasons. It's a ugly difference to dict semantics, but __setitem__ is not
  part of IInbox so it's an internal method.
- add update_new_messages_count()
- update new_messages_count in __setitem__ and __delitem__

Message:
rename .read to .mew

Update interfaces and tests