Refactoring IFileSystem

Making AssemblyNeutral and creating .Interfaces package
Changing semantics of string property to be relative
Updating tests
Changing return value to be always non-null with Exists property
I updated the XML comment of the IFileSystem to something like below:

/// <param name="subpath">Relative path that identifies the file.</param>
        /// <returns>The file information. Caller must check Exists property.</returns>
        IFileInfo GetFileInfo(string subpath);

/// <param name="subpath">Relative path that identifies the directory.</param>
        /// <returns>Returns the contents of the directory.</returns>