pingback: set 'application/javascript; charset=utf-8' request content type
postTextPlain: handle 'application/javascript' as plain text
backdata.Connection: self + its users, i.e. backdata.JS(pingback), former App.back.*
controller.Main: event handlers refactoring

remove useless refs, move handlers inside init() scope
and use them (kind of private methods)
controller.Main: global.backendEvents = handleBackendEvents()
change namespace: App.backdata => App.backend
change namespace(files): App.backdata => App.backend
App.backend.req: alias for (new App.backend.Connection).request()