Making conversations archivable

Conversation model are now able to be archived with
```@conversation.archive``` should be called on _send & archive_ call.

Conversations can be unarchived with ```@conversation.unarchive```
should be called on incoming email or when a user "opens" a
conversation.
Moving to an more generic status column instead

To get unarchived conversations now user ```Conversation.unarchived```
instead
Making some smart moves on status column for Conversations

Removing ```unarchive``` method for time being.