GPU models, performance testing.

This PR isn't yet complete, but the basic idea is to add an example of a GPU-accelerated model, and to add some basic performance testing harnesses that take the grunge work out of comparing the performance of various models and heuristics.

Still remaining to make a nice feature:
- [ ] Needs added to API docs.
- [ ] Support for collecting many trials.
- [ ] Example of calling perf_testing with reasonable parameters.
- [x] Inclusion of ``(9/8)**k`` as a callable ``Heuristic``, to avoid lines like ``perf = qinfer.perf_test(qinfer.SimplePrecessionModel(), n_particles, qinfer.distributions.UniformDistribution([0, 1]), 200, lambda up: lambda: np.array([(9.0/8)**(len(up.data_record))]))``.