Add binary search exercise

I added the first exercise: binary search.

With this exercise, I've added an @timeout decorator that will kill a test when takes too long to run. Note that it does not yet re-raise any exception thrown in the sub-process that runs the actual test, meaning that @timeout-tests now always pass as long as they run within the timeout. This should still be fixed, but I didn't find a proper way to do this yet (that will also run on Windows).

Two other minor changes:

• Fixed some whitespace errors in the generator when removing AST nodes
• GitLab CI now runs all tests it can find in exercises/*/*/testsuite.py

Closes #14 (closed)