copy librdf_node pointer on statement construction

librdf_new_node_from_node will increase the reference count
for the librdf_node to match the corresponding free of both
librdf_node(s) and librdf_statement term(s).  Incrementing
and decrementing the reference count allows the raptor_term
to be free'd only when nothing else references it.

refs #3
Merge pull request #7 from abargnesi/issue3-nodeptr-fix

copy librdf_node pointer on statement construction