Added SwiftPM support (#290)

Author: quantum2

Package.swift

@@ -0,0 +1,16 @@
+// swift-tools-version:4.0
+// The swift-tools-version declares the minimum version of Swift required to build this package.
+
+import PackageDescription
+
+let package = Package(
+    name: "NVActivityIndicatorView",
+    products: [
+        .library(name: "NVActivityIndicatorView",  targets: ["NVActivityIndicatorView"])
+    ],
+    dependencies: [],
+    targets: [
+        .target(name: "NVActivityIndicatorView", path: "Source/NVActivityIndicatorView")
+    ]
+)
+