add benchmark tests

This is just getting a start on having a set of benchmark tests. Driven out of a discussion on https://github.com/sendgrid/go-statsdclient/pull/2 (I will follow up on that pr with comparisons)

Initial results are

    BenchmarkIncrement	 1000000	      1243 ns/op
    BenchmarkDecrement	 1000000	      1242 ns/op
    BenchmarkDuration	 1000000	      1896 ns/op
    BenchmarkGauge	 1000000	      1228 ns/op
    BenchmarkIncrementGauge	 1000000	      1219 ns/op
    BenchmarkDecrementGauge	 1000000	      1210 ns/op
    BenchmarkUnique	 1000000	      1293 ns/op
    BenchmarkTiming	 1000000	      1214 ns/op
    BenchmarkTime	 1000000	      2104 ns/op
    BenchmarkPrefix	 5000000	       440 ns/op