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/main.m

@@ -0,0 +1,5 @@
+#import <Cocoa/Cocoa.h>
+
+int main(int argc, const char * argv[]) {
+    return NSApplicationMain(argc, argv);
+}