MockFileSystemWatcherFactory should throw instead of returning useless MockFileSystemWatcher

[rkoeninger]

Both overloads of MockFileSystemWatcher.WaitForChanged remain unimplemented as seen here.

[rkoeninger]

It was deemed in #373 that implementing a working MockFileSystemWatcher was too complex, so I think the default MockFSWFactory should throw NotImplementedException instead of returning a MockFSW that doesn't work.

Also, it will be necessary to allow client code to inject their own FSWFactory that they can implement their own FSW.

[fgreinacher]

I like that idea - much better for us and for our users! Thanks!