Add support fox prefix argument for prepending to all metric names.

# What this patch does

This patch adds a global prefix attribute that is prepended to all metric names that come from the reporter.

# Motivation

Some reporters (e.g. Graphite) allow a prefix to be set that is appended to all metric names. While tags are a more elegant solution to this problem it is noteworthy that at present Datadog does not have a way to enumerate all metrics with a given tag. Therefore my team uses prefixes (e.g. keen.serviceName) to segregate metrics.

# Testing

I've modified the `DatadogReporterTest` to use a prefix, but it's a bit heavy handed. I chose this method because this is how the existing global tags were tested. I am happy to revert this and create a new test if deesired.

# Thanks!

I appreciate you looking this over. Please let me know if I can do any more work to make this PR acceptable. This functionality is essential to a project I'm working on and I would love to use it as soon as I can!
Nit: looks like most of the code uses 4-space indent for line continuation. Could you change this to conform?