Add Entity Reference Field

[alexfinnarn]

It makes sense to use the entity reference field in whatever form is decided by the Backdrop community. The D7 Express layout tool used a field per region that included references to other content.

This module currently just serializes references to nodes and loads the references when the block is viewed. Using fields for the references probably has advantages, but it might not. If no performance cost, then it's probably better to use fields than a blob of data.