Switch XML parsing from REXML to Nokogiri

Main reason for the switch is that REXML couldn't parse XML attribute
values with \b character (ASCII 0x08) inside them, claiming they are
invalid. It turns out that XML 1.1 requires XML processors to parse
them, making REXML behavior a bug. Because my experience with REXML on
other projects was quite crappy (translate as "I think I would run into
other similar issues sooner or later"), I decided to switch to Nokogiri
rather than to work around this.