[WIP] [3.9] Add grunt task utilities

- [x] Add build-assets task
- [ ] Check if dependencies in www/lib/* can be added to bower (tinymce for example)
- [ ] Move www/skins/build to www/assets/builds 
- [ ] Update dependencies:bower developer task (keep or remove) ?
- [ ] Update assets:compile-less developer task (keep or remove) ?
- [ ] Update asset:build-javascript developer tak  (keep or remove) ?
- [ ] Update builder.php or add a package task (build assets, fetch php libs, remove unnecessary files (less files too), build zip file)  then remove builder.php ?