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.
Some backend unit tests for this would be awesome.