The Kindle Fire, introduced in November, nearly doubled its share over the last two months, comScore reports. Meanwhile, Galaxy Tab, which had a 23.8% share in December, is now down to 15.4%. Motorola Xoom, the number three in the category, commanded 11.8% of the market in December and has now dropped to 7%, according to the report. One notable omission: Barnes & Noble's Nook. ComScore says it con