WIP: Feature: Start to hook up a Heka metrics endpoint.

Extemely WIP - not yet ready for even testing since I have not yet published the heka collector to npm. I need to get some feedback from @kparlante to find out more about what is expected on the heka side of things.

* Add rum-diary-endpoint and rum-diary-collector-heka as dependencies.
The functionality is so simple that we don't really need rum-diary-endpoint yet. If we need a second endpoint for the metrics, then we can add the extra functionality provided here.