Add PromiseKit dependency

- Added PromiseKit dependency

Carthage/Checkouts/PromiseKit/Extensions/Photos/Sources/PHPhotoLibrary+Promise.swift

@@ -0,0 +1,24 @@
+import Photos.PHPhotoLibrary
+#if !PMKCocoaPods
+import PromiseKit
+#endif
+
+/**
+ To import the `PHPhotoLibrary` category:
+
+    use_frameworks!
+    pod "PromiseKit/Photos"
+
+ And then in your sources:
+
+    import PromiseKit
+*/
+extension PHPhotoLibrary {
+    /**
+     - Returns: A promise that fulfills with the user’s authorization
+     - Note: This promise cannot reject.
+     */
+    public class func requestAuthorization() -> Guarantee<PHAuthorizationStatus> {
+        return Guarantee(resolver: PHPhotoLibrary.requestAuthorization)
+    }
+}