Added support for ISO8601 timestamps with time zones

[sjauld]

Ref #145

http://jsonapi.org/recommendations/#date-and-time-fields
https://www.w3.org/TR/NOTE-datetime

[hypnoglow]

I'm curious why not just use time.RFC3339?

Seems to work fine https://play.golang.org/p/1Rom5WsfCYO

[sjauld]

Hmmm, I thought that I tried that and it didn't work. But yes that's a much better solution if it does work.

[aren55555]

I don't believe this works with 20180725T033557Z which is apparently also ISO 8601 compliant.

I'm more inclined to think this library should not take a stance on the actual wire format, but rather support any specification via custom types implementing the json.Marshaler and json.Unmarshaler interfaces.

[sjauld]

So maybe the support for iso8601 should be removed altogether since it is incomplete and undocumented?

[aren55555]

@sjauld IMO that's the most preferred path forward. I'd also suggest an additional README to call out how to custom types to support IS08601 or even RFC3339