Add support for ASCII representations of a DN

[arielb2]

use an existing function in 'encoding.idna' for
convert unicode to ascii using the function
according to the standard on
https://www.ietf.org/rfc/rfc3490.txt

[rthalley]

There's no need for this, it's already there.

n = dns.name.from_unicode(u'栃木.jp')
print n.to_text()
xn--4pvxs.jp.
print n.to_unicode()
栃木.jp.