create CI tests to avoid performance regressions

[blakecaldwell]

A useful travis test would be to verify that the following times don't exceed a threshold:

1. network build (NeuronNetwork._build())
2. running a basic evoked response simulation (add timing to test_hnn_core.py to avoid running additional simulations)

These checks should verify timings for embarassingly parallel (joblibs) and parallization by neuron (MPI). This issue is primarily motivated by keeping bugs from slowing down MPI parallelism.

Initially discussed in #146 (comment).