[WIP] breaking changes in 0.6

[ubnt-intrepid]

• switch the HTTP server to izanami
• remove App::with_modify_service
• tweak the error representation (reform the error representation based on failure #214)
• replace Handler::allowed_methods with Handler::metadata (Add metadata associated with the instance of handlers #215)
• switch to push-style upgrade support (switch to push-style upgrade support #217)
• polish around Responder and output abstraction