an incremental step toward better converters

this is a step toward a better converter system for configman.  The immediate benefit to this change is that the excessive quoting done by the ini handler is gone and paths to classes are completely qualified.

Some redundancy has been eliminated (there was more than one timedelta converter), and the converter section has been arranged into three sections: utility, from string and to string


there are a number of places within configman itself where option values may be converted converted more than once.  this converter function does a search to find an appropriate converter.  By "memoizing" it, I can do the search only once and thereafter get the benefit of fast lookup instead of a search.