MacBuildServer 'Install app' button

Hey,

I've added 'Install app' button into your README file. Anyone could try your sample application right in browser. Look how it's working!

To learn more, please look at: http://macbuildserver.com/github/opensource/
Merge pull request #3 from macbuildserver/patch-1

MacBuildServer 'Install app' button <- Very useful, thanks!