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
Forgot the error handling.

My bad :(
Merge pull request #51 from binford2k/features/add_adhoc_file_sharing

Forgot the error handling.
Fixed ugly tiny headers
Merge pull request #52 from binford2k/minor/style_update_for_presenter

Fixed ugly tiny headers.

Just a minor style update.
Clicking on menus now pauses the remote follower

Prior to this, if the presenter had dropped into remote mode, then you
clicked on a slide in the menu, it wouldn't change reliably.
Merge pull request #53 from binford2k/minor/fix_menu_navigation

Clicking on menus now pauses the remote follower