added update_by_query to allow user-created queries using operators other than "=", (e.g. opened_at > some_datetime).
Removing json-serilization of params

Passing serialized dictionary objects as params fails on older versions of requests. This is because requests is unable to recognise that the strings are already serialized, resulting in a ValueError:

ValueError: cannot encode objects that are not 2-tuples

If passed a dict, requests will perform the serialization itself:

:param params: (optional) Dictionary or bytes to be sent in the query string for the :class:`Request`.
.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
added missing comma
Not assuming ascii
Merge pull request #5 from amorphic/master

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