switch to using Dates.jl instead of Calendar.jl

I haven't done all the testing in the world, but so far this works pretty well.

It also happens to fix the ambiguity warnings we get when using HttpCommon and DataArrays at the same time, see https://github.com/JuliaStats/DataArrays.jl/issues/115, https://github.com/JuliaLang/HttpCommon.jl/issues/6, etc

Would love feedback!