Adds Python 3 Compatibility

So I don't have to use more.static and therefore morepath with last decade's Python ;)

One thing that still doesn't work is buildout. Zest.releaser does not work on Python 3 and therefore the buildout fails. However, with Tox that's not a requirement.

Bytestrings are used at the edges and in tests for WSGI responses . The renderer/injector still uses unicode strings internally. I first changed that, but I felt it was doing too much - I wouldn't want to write a custom renderer that returns bytestrings. So now the result of the renderer is simply encoded.

This fixes #25.