- Can now specify Column objects on subclasses which have no

- Can now specify Column objects on subclasses which have no
table of their own (i.e. use single table inheritance).
The columns will be appended to the base table, but only
mapped by the subclass.

- For both joined and single inheriting subclasses, the subclass
will only map those columns which are already mapped on the
superclass and those explicit on the subclass.  Other
columns that are present on the `Table` will be excluded
from the mapping by default, which can be disabled
by passing a blank `exclude_properties` collection to the
`__mapper_args__`.  This is so that single-inheriting
classes which define their own columns are the only classes
to map those columns.   The effect is actually a more organized
mapping than you'd normally get with explicit `mapper()`
calls unless you set up the `exclude_properties` arguments
explicitly.

- docs/tests