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.
Considering that this function has an encoding hardcoded, and that the function above is called `utf8_converter` this function should be be called `utf8_to_str()`