* Add support for iso8601 struct tag.
Currently only unix timestamps are supported for serialising time.Time objects. The JSON.API specification doesn't make a specific requirement on time serialisation format, though it does make a recommendation for using ISO8601.
This adds support for an additional struct tag `iso8601` which controls serialisation and deserialisation using the ISO8601 timestamp format.
* Update doc.go with information regarding iso8601 and omitempty tag arguments.