Fix get_metadata to handle unicode strings

get_metadata was failing with non-ascii `author` fields on Python 2. This
brings Python 3's unicode behaviour: all strings are unicode without having to
prefix them with `u''`s.

This works with every python version >= 2.6.
Normalize unicode handling only around author
Merge pull request #575 from brutasse/fix/unicode

Fix get_metadata to handle unicode strings