Created new model field OrderField

This makes ``Orderable._order`` use ``OrderWidget`` by default instead of having to override it manually in ``DynamicInlineAdminForm.__init__``.

See discussion in stephenmcd/grappelli-safe#43
I put required: False here just to match what was in DynamicInlineAdminForm's \__init__ but not sure why it needs to be – can you remember? Will this not always have a value?