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
Reviewed-on: http://review.couchbase.org/23347
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>