Tilejson-style endpoints for layers

[gravitystorm]

Each layer comes with an extent and title, policyfile and url endpoint. There's already automatically generated code for use with potlatch's vectors.xml file.

However, copying and pasting a bunch of XML isn't particularly user friendly, and the problem gets worse if we add more details to the datasets, like a description or a license or whatever.

Tilejson is a nice idea for wrapping up all the details about a tile layer into one document. We should do the same for the vector layers.

This would allow mappers to copy+paste a simple url into their editor, in order to use custom vector background layers. Without this, they need a sysadmin to configure vectors.xml, or else hunt around for a correctly configured instance.