Remove Calendar/Timezone type parameters about dates.jl

[quinnj]

I think I had a stroke of inspiration of sorts last night (coincidentally in the middle of studying for a final...). I think what I'd like to try out is converting DateTime to a fixed millisecond precision (thus removing the Instant parameter) and make the Calendar parameter implicit as well, similar to how Date is fixed at day precision with ISOCalendar. This drastically simplifies the DateTime code for 90% of use cases out there (I'm assuming).

To not lose the extensibility by being fully parameterized though, I'd like to introduce the Timestamp{I<:Instant,T<:Timezone,C<:Calendar} type, which would be used for anything requiring extensibility (other Calendar types, Timezone support, different precision levels, etc.). This way we have nice clean, simple code that would be default and Timestamp for more advanced uses.

Additional advantages of this approach are the fact that the DateTime code was pretty rigidly coded for millisecond precision anyway, and not really generalizable. A fully parameterized Timestamp type can be written with the write abstractions so that someone just needs to define a few methods and be on there way with a different precision level or calendar type. I think this will also be great because we can separate Timezone support out and have custom operation support without having to muck up all the current clean Date/DateTime clode.

[quinnj]

Closing for now.