Add support for the Mixpanel "alias" function

A call to analytical.alias_identity will now work for Mixpanel. Usage is slightly different than the already-existing KissMetrics implementation as the first parameter is ignored.

  analytical.alias_identity(nil, "the new identity")

The queueing of javascript commands is modified in two important ways:
1. "alias" is always added to the head of the queue rather than being appended.
2. "identify" is ignored (not queued) when "alias" is already at the head of the queue.

This behavior conforms with Mixpanel's recommended usage of alias and identify.
See https://mixpanel.com/docs/integration-libraries/using-mixpanel-alias.