Duplicate subscribers cannot be monitored

Checks if the new subscriber pid is already subscribed and do not add it to the list of subscribers more than once.
Using lists:keymember for add_subscriber
Merge pull request #5 from rambocoder/patch-1

Prevent multiple subscribers on a channel
Unused channels expire to fix #6
and one supervisor for all channels
changed variable name to account formq_controller
being a worker and not a supervisor
Merge pull request #7 from rambocoder/master

Expire unused channels and only have one channel supervisor
API to set global age of channels during runtime

This will change the maximum age of the channel to NewMaxAge.
Note: Channels created prior to this call will not have any effect.
Merge pull request #10 from maruthavanan/patch-1

API to set global age of channels during runtime