Add PromiseKit dependency

- Added PromiseKit dependency

Carthage/Checkouts/PromiseKit/Extensions/EventKit/Sources/EKEventStore+Promise.swift

@@ -0,0 +1,68 @@
+//
+//  EKEventStore+Promise.swift
+//  PromiseKit
+//
+//  Created by Lammert Westerhoff on 16/02/16.
+//  Copyright © 2016 Max Howell. All rights reserved.
+//
+
+import EventKit
+#if !PMKCocoaPods
+import PromiseKit
+#endif
+
+/// Errors representing PromiseKit EventKit failures
+public enum EventKitError: Error, CustomStringConvertible {
+    /// Access to the EKEventStore is restricted
+    case restricted
+    /// Access to the EKEventStore is denied
+    case denied
+
+    /// A textual description of the EKEventStore error
+    public var description: String {
+        switch self {
+        case .restricted:
+            return "A head of family must grant calendar access."
+        case .denied:
+            return "Calendar access has been denied."
+        }
+    }
+}
+
+/**
+ Requests access to the event store.
+
+ To import `EKEventStore`:
+
+     pod "PromiseKit/EventKit"
+
+ And then in your sources:
+
+     import PromiseKit
+
+ - Returns: A promise that fulfills with the EKEventStore.
+ */
+public func EKEventStoreRequestAccess() -> Promise<EKEventStore> {
+    return Promise { seal in
+        let eventStore = EKEventStore()
+
+        switch EKEventStore.authorizationStatus(for: .event) {
+        case .authorized:
+            seal.fulfill(eventStore)
+        case .denied:
+            seal.reject(EventKitError.denied)
+        case .restricted:
+            seal.reject(EventKitError.restricted)
+        case .notDetermined:
+            eventStore.requestAccess(to: .event) { granted, error in
+                if granted {
+                    seal.fulfill(eventStore)
+                } else if let error = error {
+                    seal.reject(error)
+                } else {
+                    seal.reject(EventKitError.denied)
+                }
+            }
+        }
+    }
+}