Mixpanel-py

* Form $insert_id for track and import calls (if not present) to enable server-side event deduplication.
* Retry API calls upon connection or HTTP 5xx errors. Added new retry options to Consumer classes.
* Replaced urllib2-based HTTP calls with urllib3. This allows connection pooling as well at the aforementioned retries.
* Stop base64 encoding payloads, as Mixpanel APIs now support naked JSON.
* Bug: $time in people operations should be sent in seconds, not milliseconds.

* Add `$merge` support.
* Support for overriding API host for, say, making calls to EU APIs.
* Updates to `$alias` documentation.

* Add Mixpanel Groups API functionality.

* Add `people_remove`.

* Fix bug preventing use of `import_data` with a `BufferedConsumer`.

* Catch URLError when tracking data.