BUrl: IDNA ToUnicode and ToASCII conversions.

* Since DNS are normally restricted to ASCII, the use of UTF-8 in domain
names is implemented using a "punycode" encoding.
* The request to the DNS server must be sent with the ASCII
representation of the domain name, however the Unicode one should be
used for user-visible parts.
* ICU provides an implementation of the conversion, which we use here.
* Conversion is currently done in-place and modifies the BUrl object
(this is similar to UrlEncode/UrlDecode).
* Adjust existing IDN test to make use of these methods. It's passing
now.