New: "//site.com/x" style URL support in CPURL and CFURL.

With this feature, CFURL and CPURL can now correctly transform a relative scheme URL starting with a double-slash.

For example, + CPURL URLWithString:@"//a.se/a" relativeToURL:@"ftp://b.se/c" would represent an absolute URL of "ftp://a.se/a".