Support ctime and mtime

[aaronlehmann]

Continuity should capture ctime (file creation timestamp) and mtime (file modification timestamp) in manifests, and by default, it should apply and verify these properties.

In case a particular application of Continuity needs to be insensitive to creation or modification times, it should be possible to use --ignore-attr at verify or apply time to exclude these attributes (see #27).

[AkihiroSuda]

Seems dupe of #16