Allow original the UINavigationControllerDelegate to still receive messages

Hi,

I've added a proxy object in so when using `+[ARAnalytics monitorNavigationViewController:]` the incoming navigation controller delegate will still receive messages sent to it as well as `ARAnalytics`.

I came across this issue when using playing around with some of the [new methods](https://developer.apple.com/library/ios/documentation/uikit/reference/UINavigationControllerDelegate_Protocol/Reference/Reference.html#//apple_ref/doc/uid/TP40007662-CH3-SW8) for custom transitions.
As `ARAnalytics` becomes the navigation controllers delegate the above stops working.

I've also added the `ARNavigationControllerDelegateProxy.h` as a private header file to the `CoreIOS` subspec, but as the spec version is set to 2.4 it doesn't lint in this pull request. I've also left `ARNavigationControllerDelegateProxy.h` in the top level folder as I wasn't sure if you wanted some sort of internal folder or something...!

Thanks for a great Pod by the way!!

Cheers,

Rich