Use gulp-notify, gulp-concat and gulp-clean.

- notify about errors (f.e. scss errors)
- concat all js files to app.js and vendor.js (you can define vendors in gulpfile)
- clean task (which clears the www/ directory)

Unfortunately, sourcemaps for js are turned off for now (concat doesn't support them). First we need to finish  wearefractal/gulp-concat#20 =)