Now uses nengo.builder.Builder

- Also 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
Merge pull request #47 from ctn-waterloo/nengo_builder

Use nengo.builder.Builder