Tracking infrastructure updates

• Tracking events and sending profile updates now occurs via HTTP POST by default, with data in the request body, rather than GET with data on the URL.
• Events now include the $insert_id property for deduplication support.
• Malformed URI params in attribution fields like utm_source no longer lead to exceptions.

To force tracking via GET instead of POST, use the api_method config option:

mixpanel.init(TOKEN, {api_method: 'GET'});