Add suffix parameter

We've been maintaining our own statsd client module for a while, but I want to start using this one. It covers all our requirements, except it can't add a suffix to a metric key. This PR introduces that functionality.

The use case for this is clusters with many machines. Our applications send metrics that look like this:
[env].[team].[app_name].[metric].[submetric].[host]

The prefix for the application is set to '[env].[team].[app_name]' and the suffix is set to '[host]'

Changes included:
 - Added a statsd suffix that works just like the prefix, but appends to the metric's key instead
 - Added tests
 - Updated documentation