Fix JSON decoding under Python 3.x

str(b'{}') returns "b'{}'", which is invalid JSON.
As httplib2 under Python 2.x returns str already, we
can just use content.decode() unconditionally and it
should work with both Python 2.x and Python 3.x.
Merge pull request #11 from dirkmueller/master

Fix JSON decoding under Python 3.x