- merged with_polymorphic branch, which was merged with query_columns branch

- merged with_polymorphic branch, which was merged with query_columns branch
- removes everything to do with select_table, which remains as a keyword argument synonymous with
with_polymorphic=('*', select_table).
- all "polymorphic" selectables find their way to Query by way of _set_select_from() now, so that
all joins/aliasing/eager loads/etc. is handled consistently.  Mapper has methods for producing
polymorphic selectables so that Query and eagerloaders alike can get to them.
- row aliasing simplified, so that they don't need to nest.  they only need the source selectable
and adapt to whatever incoming columns they get.
- Query is more egalitarian about mappers/columns now.  Still has a strong sense of "entity zero",
but also introduces new unpublished/experimental _values() method which sets up a columns-only query.
- Query.order_by() and Query.group_by() take *args now (also still take a list, will likely deprecate
in 0.5).  May want to do this for select() as well.
- the existing "check for False discriminiator" "fix" was not working completely, added coverage
- orphan detection was broken when the target object was a subclass of the mapper with the orphaned
relation, fixed that too.