Hosting #86 - Consume IWebRootFileSystemProvider

https://github.com/aspnet/Hosting/issues/86

Use the FileSystem from IWebRootFileSystemProvider if one is not directly specified. Because of this the package can now limit it's dependency to FileSystems.Interfaces.

Note this change modifies the behavior of `UseStaticFiles(path)`. It used to use that path as both a request path base and a physical file system base dir. It now only applies to the request path.

Depends on https://github.com/aspnet/Hosting/pull/126

@lodejard @davidfowl