First draft of the httpmiddleware proposed in #27

#27 

Fully untested!

 - [ ] Add tests
 - [x] implement the authentication
 - [ ] response format (HTTP-Accept headers) https://github.com/owncloud/appframework/pull/28#discussion_r4559901
 - [ ] create correct response (maybe through factory) https://github.com/owncloud/appframework/pull/28#discussion_r4563782