Add T0 to the symbols available through the API
Simplify astimezone() and utc_datetime() methods

My initial idea had been that all date-time methods ought to return
leap-second indicators, so that the fact of leap seconds would be
obvious to all users asking for dates, and I would not get as many
questions about apparent discontinuities when users generate close time
series that cross leap-second boundaries.

But on the twenty or thirty different occasions on which I have needed
utc_datetime() so far, I myself have *always* forgotten about the second
return value and have *never* remembered to prepare for it.  So I am
changing these methods to do the obvious thing that users will expect,
and making alternative versions of them for users who care about leap
seconds.