Don't run the checker when the process is trying to exit

When obiwan is running on complex code, it can end up trying to check functions that run as the process is exiting (presumably registered using `atexit.register()`). This doesn't work very well, because parts of the Python standard library may already have been deconstructed at that point:

```
Exception ignored in: <function WeakMethod.__new__.<locals>._cb at 0x7ff41464c730>
Traceback (most recent call last):
  File "/home/rspeer/.virtualenvs/lum3/lib/python3.4/weakref.py", line 50, in _cb
  File "/home/rspeer/.virtualenvs/lum3/lib/python3.4/site-packages/obiwan/__init__.py", line 317, in _runtime_checker
TypeError: 'NoneType' object is not callable
```

In this case, the object that can't be called because it's None is `gc.get_referrers`. I tried checking if that object was None, and then it instead crashed because `types.FunctionType` was None. All kinds of things can be None when you're exiting, it seems.

So, it seems that obiwan should also register an `atexit` which clears its `settrace` before anything else happens.