Restore Plone 4.3. support

Do not merge yet.

@thet Almost there. I opened this pull for discussion.

All tests run on Plone 4.3 with correct other versions. I should document them soon. Plone 4.3 compat portlet templates are still missing.

@maartenkling No, it should be branch=master now, when the branch has been merged. Could you fix that? Also Plone 5.0 version cfgs should be restored (and 4.3 removed), because there's a separate buildout for Plone 4.3.