fixed problem that prevented using the pod in the latest version of XCode + Swift

Hi Pierre,

Trying to use the pod in a swift project I got the error you can se on the screenshot:

![problem](https://cloud.githubusercontent.com/assets/404446/4265170/9f47fb64-3c41-11e4-8502-9b8a65c431f2.png)

To reproduce the problem:
1.- Create a swift project.
2.- Add iOS-GPX-Framework in your podfile.
3.- Add the Objective-C Header Bride
4.- Build.

The fix is just add UIKit to GPX.h

Also, I've included also some doc in the readme on how to include the pod in a swift project.

Thank you for creating the pod.
Regards, Juan.