Add http_client argument to docstrings
Adds ga_hook that is called on queries to google

This allows the user of the client to pass in a function that will be
called whenever the Google Analytics query service is called. The first
use case for this is to get accurate stats on how frequently the API is
being called.
Merge pull request #14 from alphagov/add-hook-for-ga-calls

Add hook for ga calls