implements sampling

[mcfunley]

Sampling is intended to be implemented by statsd clients. If the sampleRate provided is 0.5, for example, that implies that the client is only sending 50% of the packets and that the server should scale up the number of counts received by a factor of two. See for example the PHP client.

The existing implementation of the java client sends along a sample rate, but it doesn't actually sample. This PR fixes that issue.

[mcfunley]

The issue with the tests is that ThreadLocalRandom is new in Java 7, and this is targeting Java 6 apparently. The documentation warns about contention with Math.Random, which is why I switched it. I think the best thing to do is upgrade to Java 7, but lmk if you'd rather go the other way with this and I'll fix it.

[nemoo]

Upgrade to Java 7 sounds good. Java 6 is really kinda dated nowadays.

[coderlol]

Is this PR in a release?

[GuiSim]

Is this repo dead?

[waynr]

https://github.com/DataDog/java-dogstatsd-client appears to be a more actively maintained version of this project, I recommend submitting your patch there if you have not already done so.