Merge pull request #202 from vladimir-v-diaz/develop

Refactor test_extraneous_dependencies_attack.py.
Merge pull request #203 from vladimir-v-diaz/develop

Add test_repository_tool.py
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.