Add README disclaimer for xeditable limitation
Reorder "as function" examples to come after "as callback"
Add top-level description to make_xeditable helper in README
Allow make_xeditable pk to be specified
Remove assumption that "instance" is a model

This is to hopefully allow the first argument to ``make_xeditable`` to
be a raw value.  In this situation, we require that the field type be
specified explicitly.
Allow make_xeditable "source" to be specified

Slight oversight, since the default conditions are so automatic.  This
should allow a custom choices url to be specified without fussing with
the “extra_attrs” argument.
Add default value to field_type instead of exception
Revamp make_xeditable helper README
Correct wrong field name in README
Add fall-through options for bad configurations