Add support Swift Package Manager

Package.swift

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+//
+//  Package.swift
+//  KeychainAccess
+//
+//  Created by kishikawa katsumi on 2015/12/4.
+//  Copyright (c) 2015 kishikawa katsumi. All rights reserved.
+//
+
+import PackageDescription
+
+let package = Package(
+    name: "KeychainAccess"
+)

Sources/Keychain.swift

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+../Lib/KeychainAccess/Keychain.swift