SPM support (#1) (#44)

Added SPM support

Package.swift

@@ -0,0 +1,22 @@
+// swift-tools-version:5.1
+
+import PackageDescription
+
+let package = Package(
+    name: "SwiftyAttributes",
+    platforms: [
+        .iOS(.v8),
+        .macOS(.v10_11),
+        .tvOS(.v9)
+    ],
+    products: [
+        .library(name: "SwiftyAttributes", targets: ["SwiftyAttributes"])
+    ],
+    targets: [
+        .target(name: "SwiftyAttributes", path: "SwiftyAttributes"),
+        .testTarget(name: "SwiftyAttributesTests", dependencies: ["SwiftyAttributes"], path: "SwiftyAttributesTests")
+    ],
+    swiftLanguageVersions: [
+        .v5
+    ]
+)

SwiftyAttributes/Sources/common/Attribute+Sequence.swift

@@ -6,6 +6,8 @@
 //  Copyright © 2016 Eddie Kaiger. All rights reserved.
 //
 
+import Foundation
+
 /**
  An extension on dictionaries that allows us to convert a Foundation-based dictionary of attributes to an array of `Attribute`s.
  */