add plone 5 testing buildout and classifier.
plone 5: install (dexterity) default content types.
plone5: AT specific builders now switch to DX automatically with Plone >= 5
plone5: use EffectiveDate instead of effective attribute for cross version compatibilty.
update changelog and bump version.
Merge pull request #32 from 4teamwork/plone5

Add Plone 5 support.