Mixpanel plugin

I've just added a new plugin for tracking page views on Mixpanel, hope you find it useful.

Also performed some tweaks on the Google Analytics plugin as I realized that when binding two 'event-context-after' events (e.g.:Google Analytics + Mixpanel plugins) you receive the path value as 'event-context-after' instead of the actual path on the 2nd plugin declared. So I updated to code to get the path from **app.last_location[1]** rather than from **this.path**.

Please let me know if there is a better approach to get this path and using both plugins at the same time.