adds subscribe_multi method

This pull request adds a method to subscribe to multiple channels.   Also added some more docs for subscribe.   This is backwards compatible with past implementations, so can be for 1.1.0.

One thing that is left to do is to be able to query and return a list of available channels.  I need to see how this is done though.
personally I think that sub is not that useful. with subscribe + raw_msg, you can just handle all returning messages in one callback with if ($channel eq 'blabla') etc. anyway, it's up to you since you'll use it.