Write some tests

[njsmith]

Doesn't have to be fancy, but we really should have some kind of testing story for this project. For example, we might test:

• If you run cookiecutter, does it actually generate a project without errors?
• In the resulting project, can you do setup.py sdist, and then do setup.py bdist_wheel in that sdist?
• In the resulting project, can you actually run the test suite?
• In the resulting project, can you build the docs without getting an error?

We don't have to do everything at once -- even just a single test to make sure cookiecutter can run would be a great start that we could build on.

Here's a pytest plugin to make it easy to run cookiecutter: https://github.com/hackebrot/pytest-cookies

Here's the tests for cookiecutter-pypackage, which might be useful for inspiration: https://github.com/audreyr/cookiecutter-pypackage/blob/master/tests/test_bake_project.py