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.
```key``` & ```value_source``` are already guaranteed to be strings (containing no quotes).   We can use the "%r" form for ```an_pption.default```, though