Make performanceplatform a namespace package

Namespace packages allow multiple eggs to live within the same
namespace. This is needed for the performanceplatform package as we need
to extract a performance platform client out into a separate package for
use by the upload app.

http://peak.telecommunity.com/DevCenter/setuptools#namespace-packages
Merge pull request #57 from alphagov/namespace-package

Make performanceplatform a namespace package