Ignore xattr errors during extract if not supported by the filesystem

Closes #46.
repository: Fixed a potential infinite loop in recover_segment
check: archive metadata recovery improvements
check: More code cleanup
Update prune documentation to use new command line options,
and also to say that the time that matters is the time each
backup completes.
When -v is specified with prune, also show the timestamp of each
archive, which is helpful for debugging.  Unify this with the
formatting done by "attic list".  The spacing is chosen so that
the attic prune output is 79 characters wide when the archive
names are at most 36 characters.
Fix UnicodeEncodeError on verbose output of unicode paths

We can't assume that the stdout encoding is not less restrictive
than the filesystem encoding.
Merge remote-tracking branch 'jdchristensen/prune-docs-info'
testsuite: Fix test test_prune_repository
Improve attic check documentation

Closes #24.
Make the manifest rebuild code more robust

Try to make sure we've actually found msgpack data before feeding
it into msgpack.
check: Make item stream resync a bit more robust
check: Make sure all non-critical output is sent to stdout
check: replace --phase with --repository/archives-only
check: No progreess output to stderr during archives check
Remove explicit flush calls since line_buffering is enabled.
archive checkpoint: Reset timer after the checkpoint has been committed
check: Fix a check --repair issue and added more tests
Attempt to make the archive metadata resync more robust
Preparing release