assets folder not created on a fresh install

As discussed with @SkySymbol during the first install of Transvision, jquery is not installed because composer is set up to install the lib in web/assets which doesn't exist.

We agreed that a pre-install-cmd in composer.json is a solution we prefer over having a blank folder with a dummy file on Github https://getcomposer.org/doc/articles/scripts.md#command-events

Assigning to Vincent.
