Added --silent-overwrite flag

Fixes #1 - The bheaviour for returning non-zero exit codes is now standard but can be overwritten with the new ``--silent-overwrite`` flag
A slightly more elegant way to do this is to use the pytest tmpdir fixture:

```python
def write_file(filename, contents):
    with io.open(filename, 'w') as file_obj:
        file_obj.write(contents)

@pytest.fixture
def my_fixture(tmpdir):
    write_file(tmpdir.join('correct_1.py'), 'import json\nimport sys\n')
    write_file(...)
    ...
    return tmpdir

def test_my_thing(my_fixture):
    # use the files you created here
```

For more information:

http://pytest.org/latest/fixture.html
http://pytest.org/latest/tmpdir.html

That way you don't have to worry about the filesystem state :)