replace skiptest with nottest

[wholmgren]

This PR replaces the SkipTest decorator on the plot_test_results and write_test_results functions with the nottest decorator. It also defines a pass-through nottest decorator so that users don't have to install nose in order to run pecos.

It was easy to run the simple example and build the documentation. I ran into some issues with the pv example but didn't look closely at what was causing them. Neat project!

[wholmgren]

My text editor (like many others) automatically strips the trailing whitespace, which makes the diff a little harder to read. Sorry.

[coveralls]

Coverage decreased (-0.2%) to 45.424% when pulling 69b0806 on wholmgren:testdir into 24258aa on kaklise:master.