Implement IWebRootFileSystemProvider

#86 
Provide a FileSystem based at the WebRoot as read from the project.json.
PathPath resolves paths relative to WebRoot and prevents escaping.

Helios will want to override this service to support vdirs.

@lodejard @davidfowl 
The WebRootFileSystem would need to be settable. The idea is that Helios would just implement IConfigureHostingEnvironment to change the IFileSystem (I know it feels kinda weird making it settable)