Bug fix for --index & --no-index issue (dependency links*)

While testing pip-accel on bigger code bases / requirement sets after
implementing the previous bug fix I noticed that it would sometimes
still slow down to a crawl. It turned out that dependency links
specified in setup.py scripts were also being followed. Fortunately the
fix is very simply based on the work done in the previous commit.
Bug fix: Intercept Control-C (KeyboardInterrupt) like other exceptions