Undertow container implementation

Not sure what the protocol is for accepting container implementations, but I have written one for [Undertow](http://undertow.io/) that others might find useful. If this is something you'd be interested in taking a look at, I can open a PR with the patch

The issue names an Undertow container implementation but no repository files, tests, or entry points. Start by reviewing Jersey's protocol for accepting container implementations and the linked Undertow project; done would require maintainer agreement on the integration path and a reviewed pull request.

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