Centralize hosting and versioning of lrm-mapzen here

We’ve got a few mutant versions of lrm-mapzen floating around, copied-and-pasted in various repositories such as https://github.com/mapzen/blog/tree/ed671d516/resources/projects/turn-by-turn/demo/libs (https://github.com/mapzen/blog/issues/840).

We need to pull those changes back here, update API docs, get on Semver, and start publishing a central CDN-style URL for lrm-mapzen.js.
