Warn user that Punycode domains are nonstandard

[JeremyRand]

Per discussion with Ryan, we are going to special-case Punycode domains as NXDOMAIN in ncdns due to security concerns about homograph attacks. As such, the Namecoin-Qt wallet GUI should warn users that Punycode domains are nonstandard and will not resolve, so that users don't waste money buying domains that they can't use.