Allow extensions to load using standard script tags.
Several people have run into trouble switching to exhibit 3 because of
the change to using link tags to load extensions.  So, I've modified
exhibit (and the two main extensions, map and timeline) to work if
they are included as scripts instead of links.  This involved a little
bit of messy code on exhibit-api.js and exhibit.js , but I think it's
worth it to permit easier transition to E3.
Merge pull request #63 from karger/2013-11-01_extension_loading

Allow extensions to load using standard script tags.