Added active listening to #NNN issues

Added `hear` command that allows the bot to listen to `#NNNN` issue numbers and prints information about the ticket.
This also adds a ignored users option so that the robot does not respond to other redmine plugins that may talk in the channel.

This was only tested in hipchat environments.
Merge pull request #816 from rdohms/patch-2

Added active listening to #NNN issues in Redmine plugin