added PID management to twanager server

if "--pid" is provided as first argument, this creates a PID file in the
current working directory on startup and removes it on shutdown,
refusing to start up if that file already exists

filename includes the port number to allow for multiple instances

caveats:
* PID file not configurable; YAGNI?
* undocumented since commands' docstrings are expected to be brief
  one-liners
* not sure how well this works with threaded servers like CherryPy