PloneTestCase backwards-compatibility helper

@tisto, does this look all right to merge? It sets up a backwards-compatible PloneTestCase class that can be used for converting tests to plone.app.testing layers. I have used this to convert all the tests in the davisagli-convert-tests branch of CMFPlone.

The layer still uses ATContentTypes for now.

Known incompatibilities with Products.PloneTestCase:
- The class must be imported from here instead of from Products.PloneTestCase
- The test user has a user id different from its user name, so most tests that log in need to be updated. We could decide to change this in the layer setup.
- The testbrowser from plone.testing.z2 must be used. It must be passed the app object, and changes to the db must be manually committed before opening urls.
- No theme is installed.