Fix ajax call for file: and ftp: #73

Works for firefox.

Chrome needs to be run with --allow-file-access-from-files due to
http://code.google.com/p/chromium/issues/detail?id=47416

Opera 12 needs to set "Allow File XMLHttpRequest" with opera:config#UserPrefs|AllowFileXMLHttpRequest

I don't know for IE.

Maybe we should write a warning about this somewhere on the website with eventually a link to this page: https://github.com/mrdoob/three.js/wiki/How-to-run-things-locally

I don't know for IE