Add PromiseKit dependency

- Added PromiseKit dependency

Carthage/Checkouts/PromiseKit/PromiseKit.playground/Contents.swift

@@ -0,0 +1,27 @@
+import PlaygroundSupport
+
+// Is this erroring? If so open the `.xcodeproj` and build the
+// framework for a macOS target (usually labeled: “My Mac”).
+// Then select `PromiseKit.playground` from inside Xcode.
+import PromiseKit
+
+
+func promise3() -> Promise<Int> {
+    return after(.seconds(1)).map{ 3 }
+}
+
+firstly {
+    Promise.value(1)
+}.map { _ in
+    2
+}.then { _ in
+    promise3()
+}.done {
+    print($0)  // => 3
+}.catch { error in
+    // only happens for errors
+}.finally {
+    PlaygroundPage.current.finishExecution()
+}
+
+PlaygroundPage.current.needsIndefiniteExecution = true