Rename some Zuul config keys

zuul.route -> zuul.routes
zuul.mapping -> zuul.prefix
Add ZuulRoute as values object in zuul.routes

User can now specify zuul.routes.*.{path,url,serviceId} (with
"url" and "serviceId" mutually exclusive (and "location" is a
synonym) separately, or can use a one-one short form, like the
old style.

See gh-77
Move some of the routing logic into the route locator

We now support prefix stripping per service, e.g.

zuul:
  routes:
    customers:
      path: /customers/**
      stripPrefix: true

Will route /customers/101 -> /101 (on the customers service)

See gh-77