Add DirectoryInfo.DirectoryIsEmpty method and tests.

I had to read this about 4 times before convincing myself that it meant the same as the description below (and the code). How about, "if this is TRUE, a directory that contains subdirectories but no files will be considered empty"

What result is expected when onlyCheckForFiles is true if the specified root directory contains only directories, but at least one of the subdirectories (or their subdirectories, etc.) contains files? This is not clear from either the description or tests.