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.
The diff is not so clear but I added documentation for the args of subscribe.  As I read it below, these cou.ld be improved  however, by specifying which ones can be defaulted by the base object.