add more hooks for MapperProperty in save_obj/delete_obj, create simple FileColumn class

[Mike Bayer]

we have DeferredColumnProperty as a good example of a customized column loader, but there should be a way for these column properties to more accurately affect the save process. basically, the test case is a FileColumn that stores its data in a file. while the execute method can easily be used to load data in, the save_obj method in mapper will need to call some new method on MapperProperty that indicates "tell me if I need to add you to my property list", and then "ok, youre not in my UPDATE statement, so do what you have to to save yourself".

[Mike Bayer]

going to blue sky this for now. seems more like a proof of concept rather than something people really need at the moment. if any "we need to save column X in some goofy way" requests come up, then the change to mapper might be more warranted; but there havent been any as of yet.

[Mike Bayer]

this is ancient and nothing we'd ever do at this point.