Add support for "set" statsd metrics to statsd output

This creates a new config option for the statsd output that allows you to send data to a "set" metric, which is useful for counting unique things (users logging in to a service, for example). It requires a version bump for the statsd-ruby gem to 1.2.0 (the first version that supports the set metric).

I heard master is seeing heavy development so I applied these changes to the v1.1.13 tag for the logstash build of this patch that I'm currently running in production. Everything seems to work fine - looks like v1.1.13 uses jruby 1.7.3 and I'm running the flatjar on jdk 7u21 on RHEL5 64-bit. I did do a make flatjar-test and it didn't complain but there may have been additional tests that I was unaware of/didn't run (properly).