Introduce --move-warc-to for upload pipelines

This PR introduces `--move-warc-to DIR`, which is intended to make wpull fit better in upload pipelines like https://github.com/ArchiveTeam/archiveteam-megawarc-factory.  

Origin: @Asparagirl has a project with constraints that makes this capability useful.  Check out http://badcheese.com/~steve/atlogs/?chan=archiveteam&day=2014-05-30 around the 04:30:00 mark for more information.

The problem: it is currently not easy to determine when wpull has completed a WARC file in a series of WARCs, and we cannot safely upload a sequential WARC while it is being written.  The megawarc factory code addresses this problem by requiring atomic mv and watching a directory for new files.

`--move-warc-to DIR` allows completed WARCs to be moved off to an upload staging directory; from that point, scripts like the megawarc uploader can be used as-is.

This PR is not yet production ready; consider this more of a reality check request, i.e. "am I even doing this right?"  Here are the problems I know of:

* No tests.  I'm still coming to grips with the test suite.
* I'm not sure how this interacts with CDXes or the meta-WARC.  (As far as I can tell, it ignores them.)  The right thing to do here may be to move the CDX and meta-WARC on job completion to the given directory.
* This option could in theory be applied to any WARC recording situation.  This patch does not implement it that way, however.