Make Mixpanel.alias() send synchronously, regardless of the consumer configuration

[joeatwork]

Aliases only take affect after the alias message is received by Mixpanel, which means in asynchronous or time-shifted cases if events or updates associated with a newly created alias arrive before the alias itself, they could be attributed to the wrong user. Change the python library to always send the messages associated with alias immediately when alias() is called.

[orienwu]

78bdbe2