Only type-coerce properties to @id whose range includes URL

... as discussed with @danbri. I also restructured to code a bit to make it easier to understand.
Nice side-effect: the context is now just 20% of the size it was before.

This addresses rvguha/schemaorg#5
Merge pull request #46 from lanthaler/type-coerce-only-range-URL

Only type-coerce properties to @id whose range includes URL