Support Mixpanel's alias method natively. Closes #46

It does need both parameters, but we could pass it in as null, in which case they alias to the randomly generated id by the MixpanelAPI.

https://github.com/mixpanel/mixpanel-android/blob/master/src/main/java/com/mixpanel/android/mpmetrics/MixpanelAPI.java#L254