gevent and coverage

[ggw]

I've been working with gevent and found that coverage reports miss a lot of executed code.  So I made some small changes to both gevent and coverage to help them play nicely with each other.

So far this doesn't break any of our unit tests or cause other obvious harm, although it will disable branch coverage.  I'm only trying to get line coverage to work (at least for now).

But it seems like it was all just a little too easy.  I'm sure I'm missing something here.  Anyone care to take a look at the changes or try it out and tell me why its all wrong?

Feel free to try my changes and post any feedback here.  Especially examples where I'm not catching lines that are being executed.

Forks with my edits are available on github:
coverage source - edited trace function to ignore the unexpected change to the stack after a blocking call
gevent source - edited Hub.switch to save and restore the trace function; Greenlet.__init__ to apply the global trace function set with threading.settrace