use pytest instead of nose for automated tests

[wholmgren]

This PR switches pvlib from the nose testing framework to the pytest framework. I needed to learn pytest for another project, so pvlib is my pytest test. After a short learning curve, I've come to like Pytest a lot. Pytest seems much more powerful than nose. Also, nose is no longer maintained.

Another nice thing about pytest is that you can install the pytest-xdist plugin and run your tests in parallel across multiple cpu cores. My 4 core machine runs py.test pvlib in 46 seconds and py.test pvlib -n 4 in 19 seconds. This is more important for local testing since the CI builds are mostly limited by the bandwidth of those services.

I'll let this sit for a few days and then merge if there are no objections.