Adds Prefix Header and Xcode Project (#142)

- Prefix header includes Foundation and UIKit frameworks.
- Xcode project defines one target to build static library.
- Adds headers to Copy Files build phase.

Please check whether I'm missing any frameworks or libraries that need to be linked with the targets. I think the prefix header is a good thing, but it's also kind of duplicating some of the work done by Mixpanel.h. I would be inclined to pull that out into the prefix header, but that may be inconvenient for other users.