Implement WebDAV protocol

[ferndot]

We should implement the open WebDAV protocol (https://en.wikipedia.org/wiki/WebDAV). This is important because it will expand our horizon of cloud storage providers.

We are using https://github.com/sara-nl/js-webdav-client as our webdav client.

All work goes in the webdav branch.

[jancborchardt]

Awesome! cc'ing myself, @cosenal and @Apophenia on this. :)

[Apophenia]

Let me know when you start working on this :) I just learned that it's best to avoid the demo (http://demo.owncloud.org/remote.php/webdav) when testing WebDAV stuff. Certain WebDAV requests return unexpected results, probably because of the way it's optimized using reverse proxies.