(#16) - Split out routes into separate files

A complete refactoring. Groups all routes into separate files. There's not yet a mount point/alternate package for a lightweight version of this (i.e. something that could merge with pouchdb-express-router), but it should be possible to build one by, say, removing a few lines from the index.js file. (Or something similar that prevents duplicate code across two index.js files.)

Depends on #132 to be merged first.