Add PromiseKit dependency

- Added PromiseKit dependency

Carthage/Checkouts/PromiseKit/Extensions/Social/Sources/SLComposeViewController+Promise.swift

@@ -0,0 +1,48 @@
+#if !os(OSX)
+
+import Social.SLComposeViewController
+import UIKit.UIViewController
+#if !PMKCocoaPods
+import PromiseKit
+#endif
+
+/**
+ To import this `UIViewController` category:
+
+    use_frameworks!
+    pod "PromiseKit/Social"
+
+ And then in your sources:
+
+    import PromiseKit
+*/
+extension UIViewController {
+    /// Presents the message view controller and resolves with the user action.
+    public func promise(_ vc: SLComposeViewController, animated: Bool = true, completion: (() -> Void)? = nil) -> Promise<Void> {
+        present(vc, animated: animated, completion: completion)
+        return Promise { seal in
+            vc.completionHandler = { result in
+                if result == .cancelled {
+                    seal.reject(SLComposeViewController.PMKError.cancelled)
+                } else {
+                    seal.fulfill(())
+                }
+            }
+        }
+    }
+}
+
+extension SLComposeViewController {
+    /// Errors representing PromiseKit SLComposeViewController failures
+    public enum PMKError: CancellableError {
+        /// The user cancelled the view controller.
+        case cancelled
+
+        /// - Returns: true
+        public var isCancelled: Bool {
+            switch self { case .cancelled: return true }
+        }
+    }
+}
+
+#endif