- SqlSoup overhaul

- SqlSoup overhaul
- Added "map_to()" method to SqlSoup, which is a "master"
method which accepts explicit arguments for each aspect of
the selectable and mapping, including a base class per
mapping. [ticket:1975]

- Mapped selectables used with the map(), with_labels(),
join() methods no longer put the given argument into the
internal "cache" dictionary.  Particularly since the
join() and select() objects are created in the method
itself this was pretty much a pure memory leaking behavior.