support partial cascade rules

the cascading is not really complete, in the sense that its expecting "all" as the default, and wont really work if its not since the cascade rules arent fully accounted for in dependency.py. also, the entryways into Session and UnitOfWork are not shored up yet, as the flush() process sneaks plenty of things into the UOW that arent in the session.

for example:

from sqlalchemy import *

meta = BoundMetaData('sqlite://', echo=True)
t1 = Table('users', meta,
        Column('id', Integer, primary_key=True),
        Column('name', String(20))
)
t2 = Table('addresses', meta, Column('id', Integer, primary_key=True),
        Column('user_id', Integer, ForeignKey(t1.c.id)),
        Column('address', String(20))
)
meta.create_all()

class User(object):
        pass
class Address(object):
        pass

mapper(User, t1, properties={
                'addresses':relation(mapper(Address, t2), cascade="none")
        })

sess = create_session()
u = User()
a = Address()
u.addresses.append(a)
sess.save(u)

assert u in sess
assert a not in sess  # ok so far

sess.flush()  # a gets added to UnitOfWork via dependency processing, but not the session

sess.clear() # crashes due to partially associated object

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