Set a target in Package.swift so other projects can import Embassy (#74)

* Set swift tools version to 4.2

* Specify target and source path

* Add products to swift package file

Author: michaelnew

Package.swift

@@ -1,6 +1,8 @@
-// swift-tools-version:4.0
+// swift-tools-version:4.2
 import PackageDescription
 
 let package = Package(
-    name: "Embassy"
+    name: "Embassy",
+    products: [.library(name: "Embassy", targets: ["Embassy"])],
+    targets: [.target(name: "Embassy", path: "./Sources")]
 )