Bristol Palin, the daughter of 2008 Republican vice presidential candidate Sarah Palin, has given birth to a second child. Bristol Palin, who turned 25 in October and is a vocal advocate of sexual abstinence before marriage, tweeted Thursday that her "heart just doubled" at the birth of her new baby girl, Sailor Grace. Palin announced she was pregnant in June, a few weeks after her engagement to Dakota Meyer , a 27-year-old U.S. Marine who received a 2009 Medal of Honor , was called off after a year-long relationship. Meyer tweeted a photo of the baby girl, too, writing: "Best...