Add mock to TestCase attribute in addition to parameter list

[Sye van der Veen]

Being able to decorate a `TestCase` class and have it apply automatically to all `test*` methods is pretty cool. However, it means modifying every method to accept the new argument. To me, it would make more sense to add the mock to the `TestCase` object itself (i.e. `self`, in the tests).

In my tests I've subclassed `requests_mock.Mocker` to add a new `attr` parameter that sits beside `kw`. I've included the code below. If you agree with this idea, it would be about a 6-line change to add this functionality to `requests_mock.Mocker` itself.

```python
class Mocker(requests_mock.Mocker):
    """A version of requests_mock.Mocker that sets an attribute on `self`. See here for original:
        https://github.com/openstack/requests-mock/blob/master/requests_mock/mocker.py
    """

    def __init__(self, **kwargs):
        self._attr = kwargs.pop('attr', None)
        super(Mocker, self).__init__(**kwargs)

    def copy(self):
        m = Mocker(
            kw=self._kw,
            attr=self._attr,
            real_http=self._real_http
        )
        return m

    def decorate_callable(self, func):
        @functools.wraps(func)
        def inner(*args, **kwargs):
            with self as m:
                if self._attr:
                    # The TestCase, i.e. `self` for the `test*` method, is the first arg
                    setattr(args[0], self._attr, m)
                elif self._kw:
                    kwargs[self._kw] = m
                else:
                    args = list(args)
                    args.append(m)

                return func(*args, **kwargs)

        return inner
```