Allow optional GRAPH before graph name
Send a raptor_term to start/end a parsed graph

This means the graph_mark_handler call only works for URI terms, not
blank nodes.  It needs a new typedef and callback.

(raptor_parser_start_graph, raptor_parser_end_graph): Changed arg to
a term
Handle graph terms in parser not lexer

Replace GRAPH_TERM terminal with graph_term token that can handle all
the blank node and URI (qname or URI) variation

Call raptor_parser_start_graph / raptor_parser_end_graph with this
term.