OS X uploader (#252)

* Source for OS X uploader

* Readme for OS X uploader

* Export image

* .gitignore for Xcode project

* Remove redundant data

* Update readme instructions

* List formatting

* Remove personal copyright notice added by Xcode

* Added release build and updated readme

* point to doc cdn

clients/macuploader/Microbit Uploader/Assets.xcassets/menubar.imageset/Contents.json

@@ -0,0 +1,15 @@
+{
+  "images" : [
+    {
+      "idiom" : "mac",
+      "filename" : "menubar.pdf"
+    }
+  ],
+  "info" : {
+    "version" : 1,
+    "author" : "xcode"
+  },
+  "properties" : {
+    "template-rendering-intent" : "template"
+  }
+}