declare_namespace in __init__.py

I was just running the instructions at https://github.com/mozilla/minion#setting-up-a-development-environment and was hitting this failure:

```
running develop
running egg_info
creating minion.frontend.egg-info
writing requirements to minion.frontend.egg-info/requires.txt
writing minion.frontend.egg-info/PKG-INFO
writing namespace_packages to minion.frontend.egg-info/namespace_packages.txt
writing top-level names to minion.frontend.egg-info/top_level.txt
writing dependency_links to minion.frontend.egg-info/dependency_links.txt
writing manifest file 'minion.frontend.egg-info/SOURCES.txt'
error: Namespace package problem: minion is a namespace package, but its
__init__.py does not call declare_namespace()! Please fix it.
(See the setuptools manual under "Namespace Packages" for details.)
```

So, I believe this PR is what is missing from `minion-frontend`.