#119: fix graphing exception

Currently, when running `cylc graph`, we get:
```
Traceback (most recent call last):
  File "/opt/cylc/bin/cylc-graph", line 217, in <module>
    start_point_string, stop_point_string )
  File "/opt/cylc/lib/cylc/cylc_xdot.py", line 296, in __init__
    self.start_point_string = start_point_string
NameError: global name 'start_point_string' is not defined
```
This fixes the traceback and allows all the tests to pass.

@matthewrmshin, please review.