add purpose to location prompt

Add purpose to location prompt message as shown below:

![photo 1 2](https://f.cloud.github.com/assets/1192780/1144968/9b2016c4-1de4-11e3-8460-16347f5aa56f.PNG)

`_locationManager.purpose` is used for iOS 5 and `NSLocationUsageDescription` is used for iOS 6 and 7.