add RequestReply.preempt() method
deprecate RequestReply.abort() and reject() methods
add reservation and hold_time parameters to Requester.new_request() method
minor new_request() method doc improvement
make event-driven state transitions (#18)
fix some state transition issues (#18, robotics-in-concert/rocon_msgs#60)
improve example_requester test case
fix example_requester test case

only GRANTED requests send a PREEMPTING message to the requester.
document state transition graph (#18, robotics-in-concert/rocon_msgs#60)
remove deprecated methods from API documentation
make cancel() and preempt() events always valid (#18)