[WIP] Explicit proxies

Just a WIP for having more complex proxy behavior.

Proposed changes:

  * renames `api-stub` generator to `http-mock`, which is maybe more accurate? :bikeshed:
  * adds an `http-proxy` with example proxy: `ember g proxy api 127.0.0.1:8000`
  * updates `server` directory to include `mocks` and `proxies`
  * leaves `--proxy` behavior, but warn if used when explicit  proxies are generated
  * tests
i wonder if the generator should add the proxy to the app's package.json