Add caching of stat results

[schwa]

When dealing with many small files on the remote host, ftputil becomes rather slow because it fetches the directory listing every time it wants to stat a file/directory/link. This doesn't matter so much for larger files because then the "net data" transfer time is much greater than the time for getting the stat data. These timing issues are reversed for the case of many small files.

[schwa]

The corresponding code seems to be stable and has been moved from the branch into the trunk. Caching thus is part of ftputil 2.2b and later.