Add PromiseKit dependency

- Added PromiseKit dependency

Carthage/Checkouts/PromiseKit/Extensions/Bolts/Sources/BFTask+Promise.swift

@@ -0,0 +1,27 @@
+#if !PMKCocoaPods
+import PromiseKit
+#endif
+import Bolts
+
+extension Promise {
+    /**
+     The provided closure is executed when this promise is resolved.
+     */
+    public func then<U>(on q: DispatchQueue? = conf.Q.map, body: @escaping (T) -> BFTask<U>) -> Promise<U?> {
+        return then(on: q) { tee -> Promise<U?> in
+            let task = body(tee)
+            return Promise<U?> { seal in
+                task.continueWith(block: { task in
+                    if task.isCompleted {
+                        seal.fulfill(task.result)
+                    } else if let error = task.error {
+                        seal.reject(error)
+                    } else {
+                        seal.reject(PMKError.invalidCallingConvention)
+                    }
+                    return nil
+                })
+            }
+        }
+    }
+}