Add Timer framework.

This framework consists of interfaces and classes for managing various
kinds of timers. The Timer interface models a timer with configurable
tick intervals. The TimerCallback interface provides a means of adding
logic to the following timer events: onStart, onStop, onFinish and
onTick.

Timer implementations include a CountdownTimer (kitchen timer), and a
StopwatchTimer, which should both be self-explanatory.

The purpose of this framework is to add a means of creating a very
generalized approach to timers, in hopes of abstracting away some of
the Bukkit scheduling for easy maintenance and increased portability.

Future work:
- Port auto-start-timer
- Port boss abilities
- Port spawner
- Create 'delay-timer' as per DBO ticket request.
- Create various types of cooldowns, delays, etc.