Decouple client

This removes the client from this repo, and lays the groundwork for loading it via npm as [roots-cms-client](https://github.com/carrot/roots-cms-client).

Note that until `roots-cms-client` is published on npm, you'll need to clone the client down locally and update `client_path` in `lib/server/index.coffee` to that location in order to test.

See https://github.com/carrot/roots-cms-client/pull/1 for more details.