Refactoring of multipart handler.

Note: NOT TESTED. (Though it compiles… which is usually a good sign.)
Reorganizing and documentation.

I'm trying to keep actions at the top of the file, with contexts in the middle [as they may have
"before or after" processing on a per-controller basis], and request lifecycle stuff at the end [as
it usually defers to the application controller unless its doing something really fancy.]