change autoexpire to expire_on_commit, expire_on_rollback

[Mike Bayer]

this is for 0.5beta3, since we are in beta mode we're going to remove the autoexpire flag entirely. This needs to be updated in the documentation, gendocs, and the 0.5 migration guide.

[Mike Bayer]

flag renamed to "expire_on_commit" in 4769ea895b3312e2d27d52b45816771df8a5c5c7. the rollback behavior can be modified by saying _enable_transaction_accounting=False, turns off all the 0.5 object management.

[Mike Bayer]

Removing milestone: 0.5.0 (automated comment)