overhaul begin/commit, final take !

[Mike Bayer]

make the begin/commit work like this:

    s = objectstore.get_session()
    trans = s.begin()
    #etc
    trans.commit()

with all the requisite rollback, nesting behavior, etc. + document it.

[Mike Bayer]

committed in changeset:987. docs also updated, but not on the site yet (since release version has different behavior)