refactor converters

In preparation for adding new definition and value sources (argparse, python modules, ...) I've refactored the converters.  They're now sorted into to types: to string and from string.  Both types now have dispatch functions that will relieve clients of this module from having to find the appropriate converter manually.
it's actually indication that this method never gets used at all.  the code always invokes the instance var "converter_function" directly rather than the __call__ method.  I'll re-examine this.