Add support for getting "aware" datetime info

This adds 2 properties to commits.  Their values are derived from the
existing data stored on them, but this makes them more conveniently
queryable:

    - authored_datetime
    - committed_datetime

These return "aware" datetimes, so they are effectively companions to
their raw timestamp equivalents, respectively `authored_date` and
`committed_date`.

These datetime instances are convenient structures since they show the
author-local commit date and their UTC offset.

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