Added Tree types and get_bugdir_subscribers to becommands/subscribe.py.

Added Tree types and get_bugdir_subscribers to becommands/subscribe.py.

The Tree subclass SubscriptionType may be a few more lines to setup,
but they should be really easy to extend and will automatically keep
the longhelp and type handling in sync.

An example extension for bugdir types would be
  all -> assigned -> <user>
to subscribe only to bugs being assigned to the specified user.  You'd
have to loosen the currently strict InvalidType checking to make that
work, but the current type-tree handling would be up to the task.

Also a bit of reorganizing to hide the private functions.