Delayed build

This is the mostly complete implementation of the delayed build.

Simple example of the new syntax: http://nbviewer.ipython.org/urls/raw.github.com/ctn-waterloo/nengo/delay-build/examples/2drepresentation.ipynb
More complicated example: http://nbviewer.ipython.org/urls/raw.github.com/ctn-waterloo/nengo/delay-build/examples/controlledintegrator.ipynb

Note that this branch is a branch off of `unify-api`, which branches off of `ensemble-array`, which branches off to `reorg`. Just saying! They're all rebased linearly, so it'll be no problem to merge.

The basic user-facing change is that you now create a simulator, and run that, rather than just running the model. This makes a lot of things a lot more simple (and a few things more complex, but hopefully that'll be transparent to the user).