Ad hoc file sharing

To share files on an ad-hoc basis, the instructor can simply drop files
into `_files/share`. All files existing in this directory will be listed
when students hit the download page. Showoff does not need to be restarted,
even if you drop files in here during a presentation.
Merge pull request #50 from binford2k/features/add_adhoc_file_sharing

Ad hoc file sharing