Move statsd prefix to class level

This PR moves the global `STATSD_PREFIX` attribute to the class level to allow subclasses to modify it without resorting to hacks or injecting stuff in `os.environ`.  

This change is backwards compatible with the previous behavior.

#### Testing
1) `virtualenv env`
2) `env/bin/pip install -r requirements.txt`
3) `evn/bin/python setup.py test`