Fix : remove cache issues for css, js, templates

Packaged css and js are now suffixed with the build date.
All templates are exported to a folder named after the build date.
Streamlined the build process to copy files to piskel-website.

Isolated common part between piskel and piskel-website in a separated
template, which is now completely created by the grunt build of piskel.

Added a windows CMD script to copy the static resources to piskel-website.