Add support for BSD's stat command

This commit causes files.owner and files.group (and, notably,
not files.mode) operations to try the BSD flavor of stat if stat fails
with 'stat: illegal option'.  This allows the use of require.directory
on OS X.
Improve support for BSD's stat command

This commit causes files.mode operations to try the BSD flavor of stat
if stat fails with 'stat: illegal option'.  This allows the use of
require.file with a mode argument on OS X.
Add support for OS X's md5 utility

This change checks for LSB's md5sum utility and OS X's md5 utility (in
that order) and uses whichever it finds.
Use 64K progress dots for wget downloads
Speed up Node.js tests by caching downloads
Speed up Node.js compilation
Speed up Python tests by caching pip downloads
Make Debian package install less verbose