iOS: Updated to SDK 2.6.0 + new features

**0.7**
- Upgraded to Mixpanel SDK 2.6.0
- Allow Mixpanel customers to define new elements to be tracked in their app dynamically without having to re-release to the app store. (Really, really cool!)
- You can track the time it took for an action to occur, such as an image upload or a comment post, using `timeEvent(String name)`. This will mark the "start" of your action, which you can then finish with a track call. The time duration is then recorded in the "Duration" property.