`tools.datetime.format_datetime` was expecting aware datetime objects while
`tools.utils.xmpp_date` was returning naive datetime objects. A check has been added to
the former one, and latter one has been modified to return aware datetime objects.
Includes implementation of XEP-0082 (XMPP date and time profiles) and tests for both new plugins.
Everything is type checked, linted, format checked and unit tested.
Adds new dependency xmlschema.
fix 377