Multi-layer file and virtual filesystem support.

A layer of a multi-layer file can be singled out by name.
Apache Commons VFS strings (URI-ish) parameterize the virtual
filesystems, these are converted to OGR's vsizip and vsitar
paths.

Found GDAL bug http://trac.osgeo.org/gdal/ticket/5093 in the
process.
Merge branch 'layers'