Add benchmarks for all client commands

[nastevens]

Adds benchmarks for all client commands. Since the Bencher type is unstable in Rust (as of 1.16.0), the benchmarks are placed behind an unstable feature flag.

Benchmarks may be easily run using rustup:

rustup run nightly cargo bench --features=unstable

Preliminary benchmark results on a 2015 MacBook Pro (2.5 GHz Intel Core
i7):

test bench::bench_decr      ... bench:     137,422 ns/iter (+/- 317,266)
test bench::bench_event     ... bench:     157,708 ns/iter (+/- 279,659)
test bench::bench_gauge     ... bench:     138,378 ns/iter (+/- 254,691)
test bench::bench_histogram ... bench:     157,012 ns/iter (+/- 308,771)
test bench::bench_incr      ... bench:     183,358 ns/iter (+/- 305,843)
test bench::bench_set       ... bench:     177,160 ns/iter (+/- 308,811)
test bench::bench_timing    ... bench:     163,766 ns/iter (+/- 293,362)

[mcasper]

@nastevens Thanks for this!