Re-generate 'tests/repository_data' to include metadata unix timestamps.
Modify the expiration date format included metadata.

Convert the 'expires' field of metadata to a Unix/POSIX timestamp (previously a custom string format.)
Replace tuf.formats.py functions 'format_time()' and 'parse_time()' with unix_timestamp_to_datetime() and datetime_to_unix_timestamp().
Update affected schemas.
Update TUF modules affected by the change to the 'expires' format.
Update the unit tests affected by the change to the 'expires' format.
Update the integration tests affected by the change to the 'expires' format.
Update README.md

Update the expiration object expected by Targets().expiration
Update tuf-spec.txt

Update time format.
Update UNIX_TIMESTAMP_SCHEMA comment.
Merge branch 'develop' of https://github.com/vladimir-v-diaz/tuf into develop
Log warning if top-level metadata expires soon.
Support ISO 8601, vendor iso8601, clean codebase.
Remove extraneous example files.
Merge pull request #205 from vladimir-v-diaz/develop

Address Issue #146.  Replace custom datetime format with ISO 8601.