Auto-detect the presence of WebDAV support

[bbockelm]

The OPTIONS HTTP request is supposed a list of verbs that the server supports. Here's an example response from XRootD:

< HTTP/1.1 200 OK
< Connection: Keep-Alive
< Server: XrootD/v5.7.2
< Content-Length: 0
< DAV: 1
< DAV: <http://apache.org/dav/propset/fs/1>
< Allow: HEAD,GET,PUT,PROPFIND,DELETE,OPTIONS,COPY

If the Allow header contains PROPFIND, then the remote endpoint supports the WebDAV-version of directory listings.

After #70 and #71 are both done, change the auto configuration setting to first query the remote server for information about whether PROPFIND is supported and, if so, use that (otherwise falling back to the HTTP-based method). Keep a lookup cache, renewing every hour, of the responses for OPTIONS based on the FederationFactory class found here.