- Use new parse_datetime() method
- Drop message with error if MAM doesnt supply a timestamp.
- If the user supplys an own timestamp, save it so we can decide in the
future how to display it.
This handles every possible XEP-0082 timestamp
It has some additional options:
- Check if a timestring is a valid UTC timestamp, as required by some
XEPs (for example: XEP-0203)
- Return timestamp as datetime in UTC
- Return timestamp as datetime in localtime
- Return timestamp as epoch