Add initial support for metadata archives.

Add initial support for metadata archives.

Add a bup meta command based on the new lib/bup/metadata.py.  At the
moment, this command can record the metadata for a set of paths in a
file and then extract that metadata to reproduce the original tree
(minus any file contents).  See "bup meta --help" for more
information.

Add get_linux_file_attr and set_linux_file_attr functions to the
_helper module to handle Linux filesystem attributes -- c.f. chattr(1)
and lsattr(1).

Add a lutimes function to the _helper module to support setting
symlink times.

Add a vint module to support the metadata binary encoding.

Add some very preliminary tests for bup meta and metadata.py.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>