- make TimeWarp work with date/times:
    make the base function for dateAlign/dateShift use POSIXlt (it already does this inside the function, after converting to Date)

- make dateWarp able to take a list of specs and do aligns
  as well as shifts (get examples/tests from the S-PLUS
  bmc.misc library files bizdaySeq.html, warp.html. (Lots of
  these tests should already be in there.)

  When given a list of shifts and aligns, dateWarp should use
  the function arguments as defaults -- values should not
  propogate from one shift/align to the next.  E.g., with this
  spec:
    timeWarp(x, holidays="NONE",
             list(shift=list("2 bizdays", holidays="JPNEX"),
                  shift=list("2 bizdays")))
  the second shift should use holidays="NONE", not holidays="JPNEX".

