RCBC-27 EventMachine plugin

Introduce plugin which allows to use EventMachine as event loop for
couchbase client.

With Ruby 1.9 and Fibers it is possible to use blocking interface,
otherwise connection should be asynchronouse and connection.run should not
be used.

Change-Id: I6a5bd7c86f9ce5f3b7090b0c58bc411d6b6a52f7