[FEATURE] Add config and use composer autoloader.

Created by: dawnbreak

We use composer already to install SabreDAV. Include config composer.(json|lock) to install and manage more dependencies in future. Also provide PSR-4 autoloading for our namespaced classes, e.g. "Zotlabs". All other files and classes still need to get included manually. To regenerate autoloader maps before release use: $ composer install --optimize-autoloader --no-dev

We could also remove the whole vendor/ folder from our repository, but that would need changes in deployment and how to install hubs and needs more discussion first.