Introduce an Extension class, finally.  Making a stab at #130.

All the core functions have been moved into a core Extension, and
Namespace has been beefed up a little bit so declaring functions is
easy.
Convert extra and fonts into extensions.
Move bootstrap to an extension.
Move Compass to an extension.
Remove scss.functions!
Remove compiler's reliance on config.DEBUG.
Remove compiler's reliance on config.VERBOSITY.
Reduce tool's reliance on config.LOAD_PATHS.
Remove compiler's reliance on config.CONTROL_SCOPING.
Remove calculator's reliance on config.FATAL_UNDEFINED.
Oops, fix __all__.