Check for valid \u and \U escapes in turtle strings

(raptor_stringbuffer_append_turtle_string,
raptor_turtle_expand_name_escapes): Added checking code.
Fix DECIMAL, DOUBLE, INTEGER to match Turtle 2013.

'123.' is now an integer followed by '.' not a double
Fix statement list to match Turtle 2013 rules about '.'s

Removes 4 lexer conflicts; now expect 0
Turtle 2013 changes the way URIs are encoded :/
Mark turtle tests bad-17 and bad-18 as now good
Mark turtle tests bad-17 and bad-18 as now good
Update Turtle URI writing to match Turtle 2013 encoding

(raptor_turtle_writer_reference): Write URIs with Turtle 2013 escaped
characters; must escape #x00-#x20<>\"{}|^`