Send requests to Mixpanel from background thread.

[aaronn]

A Mixpanel DNS today took down our service because it blocked on Max Retries. Any chance these tracking events can be sent from a background thread so that in the event of an outage it doesn't hurt our service? We're using Django, so we'd need to spin up a separate worker to handle these events async– it would be awesome if this was all handled from Mixpanel-python directly.

MaxRetryError
HTTPSConnectionPool(host='api.mixpanel.com', port=443): Max retries exceeded with url: /track (Caused by NewConnectionError('<urllib3.connection.HTTPSConnection object at 0x7f387c261f10>: Failed to establish a new connection: [Errno -3] Temporary failure in name resolution'))