Compatibility with Mixpanel-swift

[timgcarlson]

Mixpanel-swift uses a protocol called MixpanelType for the values in the properties dictionary. This doesn't work with Trackable's trackEventToRemoteServiceClosure since that expects AnyObject as it's value.

I have yet to find a workaround that doesn't involve modifying the Trackable library, so in the meantime Trackable is only useable with the Mixpanel's Objective-C library (as far as I know).