First implementation of FileSystemFacade

Needed an injectable/testable way to get `IDirectoryInfo` objects from paths. Figured I would propose adding this here as I'm sure there are other people who might need it. Or at least as an attempt to start a discussion on changes needed to make it fit.

This is by no means a complete example of what a `IFileSystemFacade` might look like but it's got the methods that I needed at the moment plus a few overloads for great good.