Exclude non-runnable code from coverage

Add config file for coverage.py to exclude some common things: http://nedbatchelder.com/code/coverage/config.html

There's a few other things that are candidates for exclusion:
```
    # Don't complain about missing debug-only code:
    def __repr__
    if self\.debug

    # Don't complain if tests don't hit defensive assertion code:
    raise AssertionError
    raise NotImplementedError
```