[core] Fix Exception syntax to be forward-compatible
[core] More forward compatability

Absolute imports, print function, excetion syntax, string objects...

With this commit (and changing every instance of iteritems and itervalues
to items and values respectively), I was able to run willie on Python 3!

The iteritems and itervalues change did not make this into this commit simply
because I'm not sure if I should implement our own wrapper function in
willie.tools or use python-six for it