CB-4421 windows phone 8 exception when doing ajax to local file

Please review, I just put the minimum code to avoid the exception and have the file be read.
User still needs to pass 'www/' in front of every url to request a local file.

I had propose that we don't make this a hard requirement, user should be able to request 'index.html' or any file inside 'www/' without specifying 'www/' it works on other platforms (web, ios, android) why not make it work on wp8.

please review.