added benchmarks
added benchmarks

    $ go test -bench .
    PASS
    BenchmarkAdd_100_10_0p0    50000000          34.1 ns/op
    BenchmarkAdd_100_10_1p5    50000000          35.2 ns/op
    BenchmarkAdd_100_10_2p0    50000000          35.7 ns/op
    BenchmarkAdd_1000_10_0p0   30000000          37.4 ns/op
    BenchmarkAdd_1000_10_1p5   50000000          38.1 ns/op
    BenchmarkAdd_1000_10_2p0   50000000          37.9 ns/op
    ok    github.com/couchbaselabs/ghistogram    10.397s

couchbaselabs/ghistogram