Microsoft today said it would start selling gift cards good for purchases on its digital content market, taking a page out of Apple's iTunes playbook.Amazon.com listed digital cards today in denominations of $15, $25 and $50, and said they would be available starting Friday, but could be pre-ordered today.The cards have been in the wind for months. In May, The Verge noted that Microsoft would debut gift cards as replacements for the Xbox Points virtual currency.And in a blog last week, Microsoft mentioned gift cards in a long missive about changes to the Windows Store, the app distribution mart for Windows 8.1 and Windows 8.1 RT.According to a long
