Feature/per-metric source name

This is not a general purpose fix for [Issue #2 - extract source from metric name](https://github.com/librato/statsd-librato-backend/issues/2), but does provide one approach to provide this feature.

Metric names can have a source embedded using this format:

> source_name/metric_name

This patch will parse out the source name and pass it along to Librato Metrics with each metric that it is specified for.

This requires an additional patch to statsd to allow the '/' character to pass through to the librato backend during a flush operation.  I have a separate repo & pull request for that change.