Support for TaiInstant

[timclemons]

Would it be possible to add support for the org.threeteen.extra.scale.TaiInstant and conversion between it and the UTC time basis? The assumption would be that the user would have added the threeteen-extra library to the classpath.

[dm3]

There's probably nothing fundamentally incompatible with TaiInstant. Do you have an idea of how this would be implemented?