Use http_session parameter in Client ctor

This fixes #102 by allowing the use of a pre-authenticated http Session with gspread. Until this fix is in main, users can achieve this by setting the Client.session directly member if it is found to be None.

References:

http://stackoverflow.com/questions/21420777/how-would-one-convert-wrap-a-httplib2-instance-as-a-session

http://stackoverflow.com/questions/21467305/how-to-authenticate-with-google-spreadsheets-api-using-python-client-discovery