gaclient.Cursor: correctly handle responses without 'rows'.

A query that results in an empty resultset returns a response
object without 'rows' attribute.
gaclient.Cursor: add exponential backoff to cursor execution

If a recoverable error occurs while requesting data from Google
Analytics the Cursor object will sleep for 2^attempt + random
amount of seconds before retrying. At most 5 attempts will be made
by default, the Cursor's `attempts` paramter controls this
behaviour.