Allow changing of the Order.date_placed field

Removes the auto_now_add=True. Sets now as the default, but allows changing the date_placed field explicitly if an order is updated after it has been placed.

Fixes #1529
Make default value for Order.date_placed easier to customise.

Added set_date_placed_default method which is called by save.

As per comment in pull request #1558.
Correct spelling in docstring

As per comment in pull request #1558.
Document change to date_placed Order field
Merge pull request #1558

Conflicts:
	docs/source/releases/v1.1.rst