Plone 4.3 compatibility

When including the overrides.zcml like recommended in the docs, Plone 4.3 won't start up because of a import error:

    ImportError: No module named publisher.interfaces.browser

Since the imported class "AddMenu" which causes the trouble isn't used any more in the actual code base, I removed this line and other unused imports as well.

Additional some PEP8 fixes and a small fix in the Readme.