Marston Hefner, the 21-year-old son of Playboy don Hugh Hefner, was arrested in a Los Angeles suburb last night for allegedly assaulting Playmate Claire Sinclair. Authorities confirm to E! News that shortly before 11 p.m., cops were called to the couple's Pasadena home after receiving a domestic-violence call. Sinclair suffered minor injuries, according to police, and Hefner was taken into custody without incident. Hefner was charged with battery on a spouse and released a few hours later after posting a $20,000 bail. (Hefner and Sinclair were not married but under state law a "spouse" can be considered a cohabitant.) No...