Enhancements to WebDAV (Exchange) capabilities:

1) Automatically add / separators if not supplied by user.  Fixes
   Issue 290

2) Enable Move and Copy.

3) Enable setting a message to unread state.

4) Set authentication header for downloading and sending messages, so
   that those functions work with sites using Basic authentication.

5) Don't swallow log Exceptions.  Instead, allow Exceptions to
   percolate up to higher levels so that they can be logged into
   K9mail-errors.

6) Provide appendMessages function, so that Drafts get stored on the
   server.

7) Enable server-side message deletion, using user-selected Trash
   folder.