Feature request: add isless function to compare two Datestamps

[metanoid]

I ran into an issue where I could not find the minimum or maximum of an array of Arrow.Datestamp data, because these two functions require the existence of an isless function.

To reproduce:

import Arrow
import Dates
day1 = Arrow.Datestamp(Dates.today())
day2 = Arrow.Datestamp(Dates.today() + Dates.Day(1))
first_day = minimum(day1, day2)

Expected behaviour: returns day1 value
Actual behaviour: Error message : ERROR: MethodError: no method matching minimum(::Datestamp, ::Datestamp)
Request: export isless, minimum, and maximum functions from Arrow.jl which can handle Datestamps

What I tried:
I tried making my own isless function:

Base.isless(a::Datestamp, b::Datestamp) = isless(a.value, b.value)

However, this just leads to the new error message: ERROR: MethodError: no method matching minimum(::Datestamp, ::Datestamp)

[ExpandingMan]

On current master, the intention is that the Arrow time types should always be converted to Julia Dates types using convert. This is of course not ideal. Ultimately the Arrow time types are going to be done away with entirely, and instead the array will be a view of the underlying data which automatically returns Julia Dates types as elements.

[metanoid]

Okay, perfect, so for now I can convert all Arrow.Datestamp arrays to Dates.Date upfront!