firefox has introduced "mixed active content blocking" which means
that a web page loaded with https will refuse to load http: script
links.  At the top level this can be fixed by a user choosing suitable
relative urls for exhibit scripts
(//api.simile-widgets.org/exhibit/...)  .  However, the map and time
extensions load other scripts whose urls were hard-coded to http.
Fixed by setting these urls to https if the document url is https.
Merge pull request #41 from karger/2013-09-23_https

Modified extension scriptloaders to tolerate firefox active content blocking under https