Use stevedore to load extensions

I noticed stevedore when I updated to latest virtualenvwrapper recently.
I think it's nice to be able to do something like this:

```python
from setuptools import setup

setup(
    name='Flask-SuperExtension',
    version='0.0.0',
    description='Super Extension',
    py_modules=['super_extension'],
    install_requires=['Flask'],
    entry_points={
        'flask.ext': [
            'super_extension = super_extension',
            ],
        }
)
```

I'm just throwing this out as an idea since it would add an extra dependancy.