Fix #98. Exclude extra credit from lesson_pass_count.

I encountered the same issue as @tombusby (#98), where the remaining lessons count goes below 0:

~~~
Thinking AboutRegex
  [...]
  test_matching_set_character has damaged your karma.

[...]

You have completed 312 koans and 39 lessons.
You are now 1 koans and -1 lessons away from reaching enlightenment.
~~~

This is because `total_lessons` filters "about_extra_credit" out of its count ([L264](https://github.com/gregmalcolm/python_koans/blob/master/python2/runner/sensei.py#L264), in `filter_all_lessons`), but `lesson_pass_count` does not — until now!

This issue was encountered and the patch tested against the python2 koans. I also made the change in the python3 version, but was not able to thoroughly test it there (I did not complete them yet).