Mixpanel

* Add support for customizing JSON serialization.

* Add support for Python 3.
* Rename mixpanel.VERSION to mixpanel.__version__.
* Move from `mixpanel-py` to `mixpanel` on PyPI.
* Fix exception handling in `BufferedConsumer`.
* Fix `people_track_charge` calls without properties.

* Fix packaging.

* Fix mutable default arguments.
* Allow serialization of datetime instances.

* Add an optional `request_timeout` to `BufferedConsumer`.

* All calls to alias() now run a synchronous request to Mixpanel's
servers on every call.