Statsd changes

I've added ability to configure Namespaces in sent stats.

+ I've found and fixed a bug (IMHO) in calculated counter rate per second

TODO:
 - Update documentation
 - I'd like to add one more test validating if GlobalPrefix will really works ;), but I think right now I've cluttered the test file enough already. So 'small' refactor of tests to promote more code reuse would be in order.