add --xattr option referring to file system extended attribute containin...

...g md5sums

Inspired by a patch from Jay McCanta <J.McCanta@F5.com> who suggested
this was possible, 2013-05-14 to the s3tools-general mailing list.

This patch adds the --xattr command line and md5_xattr config file
option, specifying a file system extended attribute in the USER
namespace which contains the md5sum for the file.  If the extended
attribute is not present, the code falls back to performing local file
I/O to calculate the md5sum, as before.  This can be used to speed up
transfers by avoiding the need to calculate md5sums for all files,
assuming this has been done at least once before somehow, with the
results stored in each file's xattr.

In addition to Jay's work, this patch simplifies testing for xattr
module import success, passes the result into the Config object,
and ensures the correct namespace is used.  Also added a test to the test
suite, and the entry on the manpage.