Significant reorganization

This is a significant reorganization of some aspects of the new API of Nengo. I believe that with this reorganization, the new API is as feature-complete as the old API, so next I plan to do #45.

Summary of changes:

- Moved core objects (Signal, Transform etc.) to core.py.
- Unified connection and probe logic by making a dummy Probe object that can be connected to. So, to probe decoded output, for example, you make a decoded connection to the Probe object. Arbitrary signals can be directly probed, however, with the RawProbe object, which wraps the probe and presents the same interface as the Probe object.
- Connecting and probing is now handled by each Nengo object, and Model just calls those functions (so no registry magic is necessary). This makes it easy for custom objects (which would be like what we used to use Networks for) to define their own connections and probes; an example of this is shown in the updated #73.