Optional force sleep time between thread runs

[anselanza]

#In some cases where high framerate tracking is not required (e.g. the "main work" is being done on the application's other threads, and you just need a check for faces every half second or so), then with threaded face tracking, it can be very useful to sleep the thread for a few milliseconds in its loop, to effectively make it run at a lower framerate than the main application.

This pull request incorporates an optional int forceWaitTime into the setup function for the threaded face tracker, without breaking the more usual use case.