Initial commit of new plugin.
Merged in changes from Lex (https://github.com/lexelby/logstash-filter-throttle) from the original gist to add field substitution support to period and key.
Updated the documentation based on key supporting field substitutions.
Added tests for the throttle filter.
Merge pull request #940 from mpilone/throttle_plugin

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