Run tutorial examples in test suite

Moved tutorial Python code into separate files and updated the test runner script to:
  1. Install `bok_choy` using `setup.py`
  2. Run each tutorial example.
  3. Build the documentation.

This will conflict with #18 but will hopefully make it easier to verify the doc changes.  We will still need to visually inspect the docs to make sure that we're including the right lines in each snippet, but at least we will know that the examples run correctly.