ActiveMessaging.load_activemessaging should not load ThreadedPoller

[subakva]

Including ActiveMessaging in a Rails application will trigger ActiveMessaging.load_activemessage which in turn requires activemessage/threaded_poller.

This means that every time the Rails environment is loaded, it loads celluloid, which prints this method every time the process exits:

Terminating 4 actors...
Shutdown completed cleanly

At best, this is an annoyance, at worst, it's spawning 4 useless threads every time the Rails environment is loaded.

Given that the class isn't used unless the application executing the threaded_poller script, it should not be required in that file.

Expect a pull request shortly. :)