This option is false by default to prevent any Cross Origin issue unsupported by most webdav servers

Squashed commit of the following:

commit 1591f5e199f91abca92ba3bb46ee3aa356c976fc
Author: Romain Courteaud <romain@nexedi.com>
Date:   Fri Dec 18 15:56:42 2015 +0100

    DavStorage: rename _withcredentials parameter to with_credentials

    Add tests.
    Update documentation.

commit 33b2113ca5ac42b015264f12776326d848320022
Author: Isabelle Vallet <isabelle.vallet@nexedi.com>
Date:   Wed Dec 16 06:22:08 2015 -0800

    DAV storage: add option for cross-origin cookie propagation.