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


We could do this instead:
```python
def timedelta_converter(*args, **kwargs):
    import warnings
    warnings.warn("timedelta_converter is deprecated in favor of str_to_timedelta", DeprecationWarning)
    return str_to_timedelta(*args, **kwargs)
```
The `warnings.warn` function only prints something on stderr once per process.