Define Base.copy for ::AbstractTime

This comes up when specifying the plot bounds in Dates (e.g., in https://github.com/dcjones/Gadfly.jl/issues/354).  I suppose defining this for all `::AbstractTime` is a little dangerous as others could define their own mutable subtypes. I could easily change this into a large union if desired.  But I think, in general, there should be a copy method for these types, no?