Update Makefile

Add a new 'help' recipe to the Makefile. Make this recipe the default. This
recipe lists out all available recipes and gives a brief description of their
effects.

Add a new 'test-docstrings' recipe to the Makefile. This recipe calls
`testimony`, just as Travis does right now.

Update the `.PHONY` special target. Wrap the line so as to be readable on
narrower editors.

Add comments to the makefile to visually separate its sections.
Make .travis.yml less special

Remove the `testimony validate_docstring ...` commands from `.travis.yml`.
Instead, make Travis call a functionally equivalent command in the Makefile.

Do not install any Python packages directly from `.travis.yml`. Instead, update
`requirements-optional.txt` and make travis install the modules listed therein.
Merge pull request #931 from Ichimonji10/refactor-tooling

Refactor tooling