A WebDAV driver for chwala

Summary:
A first version of a chwala driver that allows to use any WebDAV server as a backend. It needs SabreDAV (for the WebDAV client) as a dependency. For testing purposes, running download-sabredav.sh in lib/ext handles it.

What works:

• Read/write access. Tested agains ownCloud 8.
• Operating as a driver.
• Operating as the main backend.
• Quotas.

What is missing:

• Locks. I'm using the lock database code from the existing drivers. WebDAV has its own locking mechanism that could be used.
• A better icon (using ownCloud's for now).

Reviewers: #chwala, #chwala_developers

Subscribers: machniak

Projects: #chwala

Maniphest Tasks: T247

Differential Revision: https://git.kolab.org/D6