add Field db=False flag

[Jaymon]

this will cause a a field that is passed into __init__() or modify() to be set into the class instance, but not persisted to the db. That way it can go through all the normal Field validation stuff like the normalize method if needs be, but it ultimately won't be a field in the db.

[Jaymon]

I'm not sure this is the solution I would want to implement for this problem. It would involve a lot of refactoring since everything expects Field instances to be db fields and I'm not sure it's worth the hassle for something I don't that often and when I do do it I can usually solve it a different way