Add some default binder instances with no filters

I think https://github.com/plone/plone.app.relationfield/pull/1 meant to add code along these lines here, however I can't see any evidence of it happening.

If this is sensible, then I should also be altering the demo in plone.app.relationfield to use these (so it's obvious they're there), as well as putting the handy XML snippets I have into http://developer.plone.org/reference_manuals/external/plone.app.dexterity/advanced/references.html before I loose them.