Add PROCESS_GROUP_ADDED and PROCESS_GROUP_REMOVED events

I'm using Supervisor as a major component of a PaaS system, where the process groups on a host may change frequently.  It's hard to keep my dashboard and monitoring tools up to date with these changes, because Supervisor emits no event when a process group is removed (as opposed to just stopped).  This pull request adds two new events, PROCESS_GROUP_ADDED and PROCESS_GROUP_REMOVED, so that other systems may be kept up to date with Supervisor's process group configuration.