(dom.ts): fix .down() to return a new DomJS instance
(dom.ts): fix .down() to return a new DomJS instance

Allows .down() to be chained, eg. `$$('.foo').down('.bar').css('color', 'red')

Also fixes modifying this[0] which causes previous references to break.

fabd/kanji-koohii,kanji-koohii/kanji-koohii,kanji-koohii/kanji-koohii,kanji-koohii/kanji-koohii,fabd/kanji-koohii,kanji-koohii/kanji-koohii,fabd/kanji-koohii,kanji-koohii/kanji-koohii,fabd/kanji-koohii,fabd/kanji-koohii,fabd/kanji-koohii