Added builtwith.svg/png badges to be hosted/served by gulpjs.com

To be [**properly!** served](https://github.com/gulpjs/artwork/pull/4#issuecomment-40178305), the badges should be hosted under `gulpjs.github.io` => `gulpjs.com` repository.
 The initial badge is from the one generated by http://shields.io/ as [![built with gulp](http://img.shields.io/badge/built%20with-gulp-red.svg)](http://gulpjs.com) ... so now we can use these morkdown markups (valid **after** merge!): 
- for **PNG** [![built with gulp](https://raw.github.com/cyparu/gulpjs.github.io/master/img/builtwith.png)](http://gulpjs.com) use: `[![built with gulp](http://gulpjs.com/img/builtwith.png)](http://gulpjs.com)`
- for **SVG** use `[![built with gulp](http://gulpjs.com/img/builtwith.svg)](http://gulpjs.com)`

**NOTE:** In context of `gulp/gulp` **PR** https://github.com/gulpjs/gulp/pull/405 & **issue**  https://github.com/gulpjs/gulp/issues/403 and `gulp/artwork` **PR** https://github.com/gulpjs/artwork/pull/4