Use autoload to load modules on-demand.
Add in a generic StatsD decorator.

* Logs all calls made to StatsD
* Only allows messages to be sent to the StatsD server when running in
  the production environment.
Update the documentation to use RDoc/YARD throughout.
Add in some more StatsD helpers.

* default instrumentation for Rails apps
* Stub class for use in tests
Merge pull request #7 from nature/statsd

Add in some generic StatsD helper classes/functions