Populate nengo namespace, and some other refactoring

Just to make @drasmuss's job harder! But yeah, this addresses what we're talking about in #206 and #208: now there's real stuff in there when you `import nengo`, and there's no more old API.

Also, extra bonus, I removed `PassthroughNode` (sorry @tcstewar) Now, any Node with `output=None` simply passes its signal on, like passthrough nodes did before.