Generate and parse image timestamps

[astarivi]

Addresses #37 by introducing an implementation-agnostic FileTime struct at the core module, allowing downstream to implement their own time gen and parsing mechanisms.

To implement this new time structure into the image write logic, a new function create_xdvdfs_image_with_filetime has been created, which takes the FileTime struct as an argument. create_xdvdfs_image is still available, and will default to a 0 timestamp value.

Implementation details:

xdvdfs-cli

• Uses chrono crate to generate and parse dates.
• Adds two new flags timestamp (-t) and timestamp_now (-T) to pack and compress sub-commands. These default to a 0 timestamp value if no flag is given (see Make the pack command produce a predictable image file from the same content source. #101)
• info sub-command will now print the parsed (RFC 3339), and the Unix equivalent of the image timestamp.
• exiso_compat is excluded from this implementation.
• build_image is excluded from this implementation (timestamp should be a part of the spec file?)