Unused dirs in global site-packages directory.

After installing `drfdocs` there are unused directories in `site-packages`, such as `site` and `tests`.
In `setup.py` file param `packages` should contain only `"rest_framework_docs"` entry, something like this:

```
    ...
    packages=['rest_framework_docs', ],
    ...
```

It is important also because unused directories have very common names (`site` and `tests`).
