Fix for expressions containing Keyword not successfully graphing.

[jstoker]

Hi,

When trying to graph a Keyword() instance, an exception is thrown.

Sample code:

> from parcon import *
> Keyword(Literal('a'), Literal('b')).graph()

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "parcon/graph.py", line 57, in graph
    new_list += graphable.do_graph(graph)
  File "parcon/__init__.py", line 1478, in do_graph
    graph.add_node(id(self), id(self.parser), label="parser")
TypeError: add_node() takes exactly 2 arguments (4 given)