move ajax-only options into the ajax configuration section

The `data-init-selection-url` is only necessary for ajax-loading of selection items. It should only be present when ajax is used, because the url-reversal can be harmful and slow.
Update README.md to include note regarding ajax loading URLs
Merge branch 'patch-1' of https://github.com/shezi/django-select2-forms into shezi-patch-1

Conflicts:
	select2/widgets.py
Add configuration info (INSTALLED_APPS + urls.py) to README