Address Issue #146.  Replace custom datetime string format with Unix / POSIX time format.

`tuf.formats.py`:

Modify the `expires` field of metadata to use Unix / POSIX time.

Replace `format_time()` and `parse_time()` with the following functions:
```
unix_timestamp_to_datetime()
datetime_to_unix_timestamp()
```

`repository_tool.py`:
The expiration methods now accept `datetime.datetime()` objects
```Python
repository.timestamp.expiration = datetime(2015, 10, 21, 19, 28)
```

Update comments, unit & integration tests, tuf-spec.txt, and documentation affected by the changes above.