Change `Time` initializer to have no-argument "now" option

This is a fairly straightforward change that enables a `time.Time` object to be created via a no argument initializer: ``t = Time()``.  This results in a time object with a (UTC) time corresponding to "now" (when the initializer is called) based on the `datetime.datetime.utcnow` function.  

If there's a strong preference for *not* having a no-argument constructor, this could be changed so that something like ``t = Time('now')`` does the same thing, but on multiple occasions I've found myself wanting this, and have thought it's strange there isn't an easy way to get the "now" `Time`.

cc @taldcroft