Faster method for testing notebook examples

Instead of starting up an IPython kernel and executing cells, we just convert the whole notebook to a Python script and exec it.

This speeds up test execution by about 10-12 seconds (using 4 cores) on my laptop. More importantly, it will hopefully reduce the number of times that Travis-CI just craps out while running tests.

Other minor changes:

- Moved some things in test_examples to utils.ipython
  - Should now be easy to generate Python scripts for docs
- Changed requirements-test, and pyzmq is no longer necessary
  (but jinja2 and pygments are)