Rework doctest to be less loud.
Test for failure message in doctest.
Add `skip` case and check summary counts.
Rewording of some documentation.
Merge pull request #18 from MichaelHatherly/mh/doctest-summary

Rework doctest to be less loud.

* `doctest` now returns a `Summary` object containing results of tests.
* rewording and cleaning up some documentation so that more code blocks are caught by `doctest`.