Use nengo.builder.Builder

- Also various changes due to refactoring in Nengo PR #177
- TestSimulator is now explicitly imported, as not all simulators are expected to pass these tests (but these are valid for the OpenCL backend, for now)
- Also fixes a lif_rate bug due to recent LIF changes

Merging this should wait until <https://github.com/ctn-waterloo/nengo/pull/177> is merged.