Use object instance, not static methods

This allows extending the class by creating child classes simpler: calls like StatsD::timing() are essentially equivalent to adding a "final" modifier. By creating an instance in a factory class (statsd_instance in the suggested patch), any module wishing to implement an alternate version may just implement its own factory, create a derived class, and everything will still work using the new class overrides instead of the original method when they exist.

This also makes unit testing simpler, making mocks possible.

The suggested patch also adds a new-style (PSR-0, à la Drupal 8) test suite.