Express sub-app mounting and easier node_module embedding

separated Express setup from http server setup to enable easier Express sub-app mounting;
fixed up a few places where using process.cwd() was preventing Keystone from easily being used in a fully embedded context;
added a detailed example in the [README](https://github.com/ryedin/keystone/blob/easier_embedding/README.md#mounting-keystone-as-a-sub-app-or-creating-a-node-module-out-of-your-keystone-project) (including reasonings why this is a good idea);