Add PromiseKit dependency

- Added PromiseKit dependency

Carthage/Checkouts/PromiseKit/Sources/firstly.swift

@@ -0,0 +1,39 @@
+import Dispatch
+
+/**
+ Judicious use of `firstly` *may* make chains more readable.
+
+ Compare:
+
+     URLSession.shared.dataTask(url: url1).then {
+         URLSession.shared.dataTask(url: url2)
+     }.then {
+         URLSession.shared.dataTask(url: url3)
+     }
+
+ With:
+
+     firstly {
+         URLSession.shared.dataTask(url: url1)
+     }.then {
+         URLSession.shared.dataTask(url: url2)
+     }.then {
+         URLSession.shared.dataTask(url: url3)
+     }
+
+ - Note: the block you pass excecutes immediately on the current thread/queue.
+ */
+public func firstly<U: Thenable>(execute body: () throws -> U) -> Promise<U.T> {
+    do {
+        let rp = Promise<U.T>(.pending)
+        try body().pipe(to: rp.box.seal)
+        return rp
+    } catch {
+        return Promise(error: error)
+    }
+}
+
+/// - See: firstly()
+public func firstly<T>(execute body: () -> Guarantee<T>) -> Guarantee<T> {
+    return body()
+}