Added update_by_query. Fixed passing of payload in params argument. Tidied up params for readability.

* Add update_by_query to allow user-defined queries using operators other than '=' (e.g. '>' and '<')
* .post payloads were being passed to requests as elements of  the params argument. This is incorrect. These are now being passed using the data argument as per http://docs.python-requests.org/en/latest/user/quickstart/#more-complicated-post-requests
* Tidied up params dictionaries for clarity.