ios: AGPipe: Change the URL property

Let's change the url property to be URL:

@property (nonatomic, readonly) NSString *URL

Maybe...... it should be really of the NSURL type....

Current code:
https://github.com/aerogear/aerogear-ios/blob/master/AeroGear-iOS/AeroGear-iOS/pipeline/AGPipe.h#L37