New circus command "rmprocess" and updated "incr" command

"rmprocess" is used to remove processes from a given watcher. I had to remove some processes from a watcher and "decr" was not suitable for me because it removes the oldest processes. With the new command you can pass the list of pids to be removed.
"incr" command is updated to include the new pids in the response.
It is not backward incompatible in the sense that *numprocesses* still exists. It is backward incompatible in the sense that *pids* appeared as a new return parameter.

 If you look at watcher.py:1049 
raise gen.Return(res) changed to raise gen.Return({'numprocesses': res, 'pids': pids})