Add Swift Package Manager Support (#49)

* Add test package.swift support

Package.swift

@@ -0,0 +1,30 @@
+// swift-tools-version:5.3
+import PackageDescription
+
+let package = Package(
+    name: "BiSON",
+    defaultLocalization: "en",
+    platforms: [
+        .iOS(.v9)
+    ],
+    products: [
+        .library(name: "BiSON", targets: ["BiSON"])
+    ],
+    targets: [
+        .target(
+            name: "BiSON",
+            path: "src",
+            exclude: [
+                "Makefile.am",
+                "Makefile.in",
+                "emhashmap/LICENSE",
+                "emhashmap/README.mkd",
+                "emhashmap/Makefile.in",
+                "emhashmap/Makefile.am",
+                "emhashmap/runtests.sh",
+                "emhashmap/tests/tests.c"
+            ],
+            publicHeadersPath: "./"
+        )
+    ]
+)

src/module.modulemap

@@ -0,0 +1,8 @@
+module bson {
+    header "emhashmap/emhashmap.h"
+    header "bson_object.h"
+    header "bson_array.h"
+    header "bson_util.h"
+
+    export *
+}