Support for the Unpoly 2 protocol

[triskweline]

Thank you for maintaining this package!

This is just a heads up that Unpoly 2 is going to be released this week.

Unpoly 2 comes with several extensions to the optional server protocol that this repo implements. All protocol changes are backwards compatible, so Unpoly 2 users can keep using this package.

You may want to consider whether you want to implement some of the newer features like X-Up-Events, X-Up-Mode or X-Up-Accept-Layer.

If you're planning to stick with the current feature set, that's cool too, of course.