ARC Compatibility

This release makes the Mixpanel library ARC compatible.

If you were previously using this library in an ARC-enabled project and you were using the -fno-objc-arc flag on all Mixpanel files, you can now remove that flag.

If you are using Mixpanel in a project that doesn't have ARC enabled, you will have to add the -fobjc-arc flag to all the Mixpanel files.

You can add and remove compiler flags on a per-file basis in XCode, in your project settings under Build Phases > Compile Sources.