timestamp generated from build task and added to about page

addresses issue #7716
when running from source, the about screen will show "Build Timestamp: Running from local source."
when running from dist, the about screen will show "Build Timestamp: Wed, 28 May 2014 04:33:48 GMT"

updated build-config grunt task to add a timestamp to the dist/config.json.
added a placeholder to the config.json files that are maintained in src.

I took a guess that updating a src file every build script run would cause too many changes for source control.  Hopefully I'm on the right track here.  I'm not sure exactly how dist/ is packaged and deployed, so my testing was done by moving the dist/config.json into src/config.json and running brackets.