Add PromiseKit dependency

- Added PromiseKit dependency

.gitignore

@@ -0,0 +1,18 @@
+## Build generated
+build/
+DerivedData/
+
+# Swift Package Manager
+#
+# Add this line if you want to avoid checking in source code from Swift Package Manager dependencies.
+# Packages/
+# Package.pins
+# Package.resolved
+.build/
+
+# Carthage
+#
+# Add this line if you want to avoid checking in source code from Carthage dependencies.
+# Carthage/Checkouts
+
+Carthage/Build