Closes#271
This PR adds track and path rendering at lower zooms than currently provided, and also achieves near-parity with openstreetmap-carto on track and path rendering. A previously-abandoned attempt, with significant discussion, was #1169.
This PR adds unit tests for the transportation layer, and fixes several bugs in the unit testing code. It verifies the correct zoom association to the various highway classes, and verifies that updates to highway attributes are populated.
Additionally, the documentation for unit tests in the CONTRIB guide is updated to note the specific make commands for developers to run.
