Fixes for changes to Signals

Fixes for changes in <https://github.com/ctn-waterloo/nengo/pull/183>.

Also changed a thing in `ast_conversion.py` and `sim_ocl.Simulator.run_steps` to fix issues with running Nengo tests. Right now, they all pass except for 7 for `sim_npy` and `sim_ocl`; there are some errors with some things being shape `(x,)` instead of `(x,1)`, and the rest are due to `MultiProdUpdate` having no `xT` attribute.

Note that this PR depends on PR #47; that is, this branch is based on the `nengo_builder` branch. That branch has been rebased to `master`, so once that's merged, rebasing this to master should be trivial.