Improves the item updating methods

putItemAttribute moved from the Scrapped.sync object to Item.prototype.

Adds a new Item method "undoChanges" which takes the original attributes
object passed to putItemAttribute, as well as the JSON of the item
returned from the server, and updates all item's observables (the keys
in attributes) with their correct data in the JSON. (It's called in
$.ajax's error callback, when changes need to be reversed.)

Also doesn't send reminders packed in Item JSON anymore, since they
aren't constructed that way anymore.