Suggest use of httplib2 v0.9.2 in readme if using python > 3 #300

Python3.x implementations of the client, passes bytestring to httplib2 header encoding method. Httplib2 < 0.9.2 does not support this.

Documentation should make a note that implementations in python3 should explicitly install httplib2 0.9.2. PyPi index targets 0.8.0, which is no bueno for those unaware of this issue.