Add support for webdav

[mkasztelnik]

There is a danger that dropbox integration will be rejected by Security Team during Security Audit. They advise us to add webdav instead of dropbox. We should evaluate how support for webdav can be added. And make dropbox optional.

First found resource for webdav in RoR environment is dav4rack and here is an example how it can be used with RoR and devise