make query generative assertions overrideable

[Mike Bayer]

this means, have the _generative decorator take strings, not function refs as arguments, have the assertions use single-underscore names. alternatively, provide a generative method query.disable_assertions(*names) to ease the task of constructing custom query classes.

[Mike Bayer]

c30cd4a6ab8b7884aa6a3ac9ac1141ede895191a.

[Mike Bayer]

Removing milestone: 0.5.6 (automated comment)