Support fanout for in-memory transport

Since the memory transport's 'broker state' is global. Here's a
quick implementation of fanout for the memory transport. We at
Nova/Oslo project were using this support in the really old
versions of kombu and would love to re-enable those test cases
that use in-memory transport.

Fixes Bug 195
Merge pull request #196 from dims/master

Support fanout for in-memory transport