Rewrite Updatehandler

[JedWatson]

Getting ready for custom field types, and as hi lighted in #526, we need to fix the "inversion of control" problems with the UpdateHandler.

It currently has hard-coded logic branches to handle the requirements of different built-in field types; these will need to be refactored into a format that field types can register hooks with (or respond to with specific, optional methods).

We're halfway there with things like the getRequestHandler methods on some fields, we just need to finish cleaning it up.

[grabbou]

As it's not major issue, we can fix that after finishing #526. That's why I suggest switching to 0.3 milestone. What you think about that?

[creynders]

updateHandler will be dropped entirely