support loading from file:// URLs

[willscott]

For use in phonegap projects where no local web server is running.
We support this in tests. Probably worth supporting it in the library itself as well.

[agallant]

Is the idea that this would require running/building freedom.js in a specific way ("debug")? Also, I noted that a bit after this issue was filed, @bemasc added gopher support (fc7809a) noting Cordova (which I believe is related to Phonegap). Did that resolve this issue, or do we still need to have file:// support?