Add support to timeEvent for measure time duration the events

[nicobytes]

For example:
In SingInPage:
mixpanel.time_event("Log Out");
In LogoutPage:
mixpanel.track("Log Out");

Result: Session length tracking

https://blog.mixpanel.com/2015/06/22/community-tip-session-length-tracking/?discovery=categories

[samzilverberg]

it actually already exists since version 2.2.0 2016-07-31.
but i forgot to document it in the readme.
will quickly add to readme.
window.mixpanel.timeEvent(eventName, onSuccess, onFail)

[nicobytes]

Hey! Now in Ionic-native there is support for timeEvent danielsogl/awesome-cordova-plugins#1536