Made EDITOR/VISUAL be a command line like in hg/git.
User interface improvements.

- Changed how the help function is implemented to allow global overriding.
- Added support for token normalization to implement case insensitive handling.
- Added support for providing defaults for context settings.
Added example for token normalization.
Added an example for help parameter customization.
Cleaned up context defaults and documented them.
Updated complex example to use context defaults
defaults -> settings
Added tox support so it's easier to run test locally
Merge pull request #124 from zyegfryed/master

Added tox support so it's easier to run tests locally