Refactor to break objects out of builder and assembler.

- Moves objects into object specific files.
- Keeps inline with upstream Nengo changes.
- Remove unnecessary code, neaten the remove pass node functionality
  - Requires ctn-waterloo/nengo-temp#446
Upstream changes to Nengo.

- `objects` Neurons now in `ensemble`.
- `decoders` => `solvers`
Merge pull request #100 from ctn-waterloo/builder-refactor

Upstream changes, refactor to break objects out of builder and assembler