Create API bundle

 * Removes the generic Middleware bundle and creates an API bundle in its place
 * Allows for development of other bundles
 * Allows for non-bundle code

Merges into `feature/event-sourcing` master PR.
Not sure if I like this... Can't really think of strong arguments against it bar preference, just an icky feeling of having a bundle directly in `src`. Also was thinking about using `Surfnet\Stepup` as the domain (not `Surfnet\StepupMiddleware\Stepup`, which would be a tad different... could just be an additional autoloader line... hrrrms.... Any particular reason on why it's done this way?