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.
What's wrong with this? On the next 3 lines after the doctstring, it takes care of this thus saving be from having to type `get_from_string_converter(type(100))`