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.

- remove extra _load_fixtures() method.   using the classbound version as the fixtures
seem to have everything classlevel at the moment (wondering if we want to change that
for non-classlevel setup/teardown)

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

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

outside of "sqlalchemy" and under "test/".

Rationale:

- coverage plugin works without issue, without need for an awkward
additional package install
- command line for "nosetests" isn't polluted with SQLAlchemy options

[ticket:1949]

- more docs for engine, pool, DDL events
- update DDL sequences documentation to use events
- update DDL() docstring to refer to execute_if()
- document parameters for DDLElement.execute_if()
- add retval=True flag to Engine.on_before_execute(), on_before_cursor_execute().
wrap the function if retval=False, check for appropriate usage of the flag, add
tests.
- remove ScopedSession.mapper and tests entirely
- remove ExtensionCarrier and tests
- change remaining tests that use MapperExtension to use MapperEvents

called if a Session is already present (checks only the
current thread) [ticket:1924]

relationship(), to eliminate confusion over the relational
algebra term.  relation() however will remain available
in equal capacity for the foreseeable future.  [ticket:1740]