New throttle filter plugin to be able to throttle events over a time period.

The throttle filter allows you to throttle events over time by specifying a lower bound, upper bound, key, and period. The events are counted and "throttled" below the lower bound and above the upper bound. This is extremely useful when using Logstash for email alerting because it allows you to get, for example, one email per hour for an issue rather than every few minutes or seconds as the issue occurs.

Code is documented and test cases included.