only escape Edge#node_one and Edge#node_two when output

Theory#adjancy_matrix, Theory#incidence_matrix, and
Theory#distance_matrix all depend on Edge#node_one and Edge#node_two to
access other nodes in the graph. Because GraphViz#get_node expects an
unescaped node id, but node_one and node_two were escaping, this caused
nil to be returned from GraphViz#get_node calls.

The only place that required escaping for Edge#node_one ad Edge#node_two
was in Edge#output.

So escaping was removed from Edge#node_one and Edge#node_two, but added
to their usage in Edge#output.

This fixes #83