Make cnx-epub python3 compatible

"next" for generators have been renamed in python3 to "__next__".  I
just changed it to use a for loop instead which works in python2 and
python3.

Python3 is does not allow mixing str (python2 unicode) and bytes
(python2 str).
Merge pull request #9 from Connexions/python3

Make cnx-epub python3 compatible