Issue 71 - Install missing dependencies, plone.app.relationfield & plone.app.intid

Dexterity documentation instruct to add plone.app.relationfield to metadata.xml
http://plone.org/products/dexterity/#relation-support-no-longer-included-by-default
That installs plone.app.intid too, without this package you hit the bug reported at issue #71