Add support for the allOf keyword

[pcpiela]

Hi all, in this proposed update I have tried to add support for the "allOf" keyword. Although, I consider myself a novice in both JSON-schema and the brutusin code base, I do have a need for the "allOf" keyword in a project I am working on; and wanted to share/contribute my changes back to the community.

I have found the brutusin/json-forms project to be very useful, better than other alternatives; and would like to what I can to move the development effort forward.

Peter