Ulta Beauty's (ULTA) CEO Chuck Rubin resigned Thursday morning, sending shares falling 11% in the stock market today. Rubin stepped down to take up the reins of Michaels Stores, North America's No. 1 arts and craft retailer. His departure comes after CFO Bruce Hartman resigned in October. The vacant CEO and CFO positions prompted Credit Suisse to downgrade Ulta to "neutral" from "outperform." "It is difficult for us to have an 'outperform' on a