Remove dead code protected by RAPTOR_JSON_WRITER_DATATYPES
Declare xsd datatypes uris in raptor_world
Use XSD namespace, datatype URIs from raptor_world
Allow reading Turtle numeric strings

(raptor_ntriples_parse_term): Gains a allow_turtle argument.

(raptor_parse_turtle_term_internal): Added to implement numerics:
xsd:integer, xsd:decimal and xsd:double.

(raptor_new_term_from_counted_string): Update code and docs to note it
allow reading Turtle numeric literals.