Respect presence and absence of modifiers (large refactoring).

Fixes:
  * https://github.com/marquete/kibo/issues/3
  * https://github.com/marquete/kibo/issues/9
Remove unnecessary assertions.
Coerce keys into strings when registering (more refactoring).

Fixes https://github.com/marquete/kibo/issues/11.
Add kibo.min.js (http://marijnhaverbeke.nl/uglifyjs).