feature: Allow routes to be instantiated.

* If a route file exports a function, instantiate the function to create the route. Very useful for testing.

fixes #42
Merge pull request #43 from shane-tomlinson/issue-42-initialize-route

feature: Allow routes to be instantiated.
Refactor: `include.js` serves RumDiaryJSClient

* Add the `/metrics` endpoint.
refactor: Extract the metrics collector from POST-metrics.
Add a test for POST-metrics.
Convert POST-metrics to be an instantiable route. Add tests to ensure data is passed on to collectors.