Add a "path" option for a namespaced peer server.

Addresses #78.

My blockers:

- Not sure `path` is a great name for this option. It's also a bit weird because you have to pass in only the middle part of the path, which makes it a bit hard to explain in the docs. (e.g. `my_namespace` instead of `/my_namespace` or `my_namespace/`)
- I actually think it'd be nicer if you can just pass in a fully-formed path (e.g. `https://peerjs.com:8000/my/path` or `https://peerjs.com/my/other/path`)

@ericz: What do you think? Probably would be good to look at what other libraries do here.