Added basic <picture> support.
Refined schema for <picture>.
Support img@srcset and img@sizes.
Marked img@srcset & img@sizes as HTML5-only.
Enable requirement for width if sizes specified.
Added stub for "source size list" datatype.
Added stub for "image candidate strings" datatype
Minor tweek to descriptive strings.
Added VnuBadAttrValueException.
Simplified the schema parts for srcset.

The width-required-or-not datatype complexity is moving to Assertions.java.
"IRI reference" -> "URL"; "abs. IRI" -> "abs. URL"
Use .THE_INSTANCE for datatypes, not new foo().
Put srcset datatype association in Assertions.java
Checkpointing srcset datatype checking.
Minor update to sizes datatype stub.
Make URL checker optionally report bad URL value.
Trim URLs to 50 chars in msg from URL checker.
Added ImageCandidateURL.
Use ImageCandidateURL when checking srcset URLs.
Checkpointing srcset datatype checking.