[patch] context processors for fields and manipulators
The manupulator and the model currently have no way to acccess anything in the request. This makes it impossible (using admin or generic views) to do something like an auto-filled author field where the author is the current user. This patch creates a new type of context that is passed in to a manipulator's __init__ method. The context is normally a ManipulatorContext object, but can be any dict. By default, ManipulatorContext gets a list of context processors by calling f.get_context_processors() on each field in the model. This context object is passed to get_manipulator_new_data when an object is saved via the manipulator.
This could probably be used for callable choices and callable defaults for fields. Also, this should make everything in #1132 possible.
superseeded by #1268