Refactor

- Add explicit `key_value` support for statsite. **Warning**: this means a gauge call no longer is a key_value call on statsite; you need to call `StatsD.key_value` instead.
- Simplified the packet generation.
- Moved some code around, and actually made private methods private :)

I also added some aliases because people were confused by some of the names:
- Add a `StatsD.counter` alias for `increment`.
- Add a `StatsD.timer` alias for `measure`.