feat(backend): support for upload/download to ownCloud server

Rough base support for ownCloud includes:
* ability to configure ownCloud instance in backend/config.js
* automatically uploads files to ownCloud when arriving to backend
* files downloaded from ownCloud before serving to frontend through backend API
* support for secure SSL connection with ownCloud
Merge pull request #19 from ghachey/owncloud-support

feat(backend): support for upload/download to ownCloud server