Ignore metadata directories during extension installation

This is a (perhaps debatable!) pull request to ignore filesystem metadata directories, which might be included in extension zip files, during extension installation. For now, there is just one ignored directory, __MACOSX, which is included in zip files created by the Finder. With this patch, these directories are ignored, and consequently don't affect the computation of the "common prefix" of the entries in the zip file. 

CC @dangoor