• 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.
• Replace urllib2 HTTP library with urllib3. This enables connection pooling as well as 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.