Got `git annex export` working to webdav and rsync special remotes. Tested
exporting to the Internet Archive via S3, and to box.com via webdav. Both
had little weirdnesses in their handling of the protocols, which were
worked around, and it's quite nice to be able to export trees to those
services, as well as Amazon S3.

Also added connection caching for exports, so S3 and webdav exports only
make one http connection, instead of one per file.

Had to change the format of `git-annex:export.log`; the old format didn't
take into account that a repository can export to several different remotes.

Today's work was supported by the NSF-funded DataLad project.
