Allow Time/Date objects for timestamps in events

Closes #30

Anything that responds truthfully to #kind_of?(Date) will have #to_time called
on it.  Ruby's DateTime class is a subclass of Date.  The #to_time method is
expected to return a Time.

Anything that then responds truthfully to #kind_of?(Time) will have #to_f
called on it, which is supposed to yield a Float of seconds since Epoch.  This
is converted to an Integer of milliseconds since epoch of the Orchestrate API.