Support Mixpanel's alias method natively. Closes #46

If the user tracks anything beforehand, we call `mixpanel.track` which uses Mixpanel's auto generated ID. When they alias the first time, I believe we should keep their original ID so use null  as the second argument. 

Additionally, iOS has opted to remove the alias method for simplicity, and ask the user to call .alias on the direct Mixpanel instance since its only an artifact of  Mixpanel. We need to decide what to do here. I don't know if this is possible though in Android since getting the `MixpanelAPI` instance to alias in the first place requires passing the context and the Mixpanel token which is something we don't want the user to have to use. Thoughts?

Also - @DirtyAnalytics just had a call with XOgrp where they are wondering how to alias properly, so I believe we should push this fix with the second parameter as null, and then consider if we can remove the alias method.