more liberal handling of "format": "uri"

Examples shown in docs are often not absolute URIs, but may just be a document or fragment name. Unfortunately this means almost anything passes the test.
the loader can be changed at any time (not just in the constructor)
bugfix: resolutionScope should change when descending into a $referenced schema
more detailed error messages for the anyOf and oneOf keywords
add a couple of examples
tweak test to catch the bug that was fixed in 0ae6c3541eb20567c20ad8b57bd7236ca7795692