Switch to manual statsd message parsing.

Hi there, 

This is a patch which fixes #44.

Before (using the regexp)

    BenchmarkMessageParser	  200000	      8574 ns/op

After (manual parsing)

    BenchmarkMessageParser	 5000000	       552 ns/op

I also added support for 'm' and 'h' metric types as described in the [specification] (https://github.co//b/statsd_spec)

I also added a couple tests.

'Hope that help :)