- merged loader_strategies branch into trunk.

- merged loader_strategies branch into trunk.
- this is a wide refactoring to "attribute loader" and "options" architectures.
ColumnProperty and PropertyLoader define their loading behaivor via switchable
"strategies", and MapperOptions no longer use mapper/property copying
in order to function; they are instead propigated via QueryContext
and SelectionContext objects at query/instnaces time.
All of the copying of mappers and properties that was used to handle
inheritance as well as options() has been removed and the structure
of mappers and properties is much simpler and more clearly laid out.