don't create a history entry when an object in a backref has changed

The code that determines whether an object in a relation has been added/removed does not take into account that that relation may be a backref. If the relation is a backref, then nothing on the current table is changing, and therefore no history entry should be created.
- clean up this test (really we don't even need this, it's not testing much)
- for the moment, put a catch in it to see if we can trap that issue
on jenkins
can remove this, issue is fixed
Merged in goodscloud/sqlalchemy (pull request #2)

don't create a history entry when an object in a backref has changed
Merge branch 'master' of bitbucket.org:zzzeek/sqlalchemy
changelog
changelog for 0.8
Forgot final check that a delete in a backref also doesn't create a new version

follow up to https://bitbucket.org/zzzeek/sqlalchemy/pull-request/2
migration for [ticket:2751]
- rework PropComparator.adapted() to be PropComparator.adapt_to_entity(),
passes in AliasedInsp and allows more flexibility.
- rework the AliasedClass/AliasedInsp relationship so that AliasedInsp has
all state and functionality.  AliasedClass is just a facade.
[ticket:2756]
turn this requirement back down as the simpler serialize works better now
remove double methods
fix up the isolation level docs which were a mess
Merged in goodscloud/sqlalchemy (pull request #3)

Forgot final check that a delete in a backref also doesn't create a new version