RFR: 416: Expose committer date in CommitMetadata

[Erik Helin]

Hi all,

please review this patch that removes the `date()` method in `CommitMetadata` and adds `authored()` and `committed()`.
For Mercurial commits `authored()` will equal `committed()` since Mercurial does not support different author and
committer dates.

Testing:
- [x] `make test` passes on Linux x64
- [x] Added a new unit test

Thanks,
Erik

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Commit messages:
 - skara-416

Changes: https://git.openjdk.java.net/skara/pull/664/files
 Webrev: https://webrevs.openjdk.java.net/skara/664/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-416
  Stats: 110 lines in 29 files changed: 47 ins; 3 del; 60 mod
  Patch: https://git.openjdk.java.net/skara/pull/664.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/664/head:pull/664

PR: https://git.openjdk.java.net/skara/pull/664