[WIP] removal of duplicate about attribute in collections rendering

This PR try to address https://github.com/flack/createphp/issues/29 and https://github.com/flack/createphp/issues/44.

The idea is to add a *noautotag* tag in twig to allow the user not to render the enclosing html element automatically. By default, this tag is not set and the enclosing div is rendered with the rdf attributes. To avoid the about attribute duplication, `Collection::renderAttributes`checks if the parent is currently rendering and if yes removes the about attribute. 

Do not merge this PR yet, some changes are still to come:
* new test without noautotag in the twig template
* new test to detect the case where an about property is removed 
* README update with noautotag information