fetched when joining/joinedloading across a many-to-many
relationship to a single-table-inheriting
subclass with a specific discriminator value, due to "secondary"
rows that would come back.  The "secondary" and right-side
tables are now inner joined inside of parenthesis for all
ORM joins on many-to-many relationships so that the left->right
join can accurately filtered.
[ticket:2369]

- paths now store Mapper + MapperProperty now instead of string key,
so that the parent mapper for the property is known, supports same-named
properties on multiple subclasses
- the Mapper within the path is now always relevant to the property
to the right of it.  PathRegistry does the translation now, instead
of having all the outside users of PathRegistry worry about it,
to produce a path that is much more consistent.  Paths are now
consistent with mappings in all cases.  Special logic to get at
"with_polymorphic" structures and such added also.
- AliasedClass now has two modes, "use_mapper_path" and regular;
"use_mapper_path" is for all those situations where we put an AliasedClass
in for a plain class internally, and want it to "path" with the
plain mapper.
- The AliasedInsp is now the first class "entity" for an AliasedClass,
and is passed around internally and used as attr._parententity
and such.  it is the AliasedClass analogue for Mapper.

become an externally usable package but still remains within the main sqlalchemy parent package.
in this system, we use kind of an ugly hack to get the noseplugin imported outside of the
"sqlalchemy" package, while still making it available within sqlalchemy for usage by
third party libraries.

inheritance subclass are now tracked, and
are used to eliminate the additional WHERE..
IN criterion normally tacked on with single
table inheritance, since the join should
accommodate it.  This allows OUTER JOIN
to a single table subclass to produce
the correct results, and overall will produce
fewer WHERE criterion when dealing with
single table inheritance joins.
[ticket:2328]

- move testing.TestBase to test.lib.fixtures
- massive search and replace

local cls.Basic, cls.Comparable base class so that there is no ambiguity
or hash identity behaviors getting in the way of class registration.

all referncing tests to not use globals
- tests that deal with pickle specifically load the fixture classes
from test.lib.pickleable, which gets some more classes added
- removed weird sa05 pickling tests that don't matter

access to the cls/self.tables/classes registries
- express orm/_base.py ORMTest in terms of engine/_base.py TablesTest,
factor out common steps into TablesTest, remove AltEngineTest as a
separate class.   will further consolidate these base classes