Allow sub-url mount point

Hi John,
Please can you review this pull request? e.g. Is it appropriate to use "@" in the constructor and throughout the code?
The requirement is to allow XmlHttpRequest to replace the original node.js that we're still using.

To do that I need to be able to create a mount point that uses a sub-url of a website, rather than the root of the website. For our purposes this means the listings file will be smaller and that we can separate the doppio-aspects of the web app, from all of the other content stored on the web site.
The original BrowserFS tests pass in Chrome/Safari/FF but I have not yet tested this code with a non-empty prefix_url value. Thanks,Lawrence.