The Texas gunman who murdered 19 children and two teachers posted on Facebook that he was going to shoot up a junior school about 15 minutes before his rampage, Governor Greg Abbott said on Wednesday, as harrowing details about the attack continued to emerge. The gunman, identified as 18-year-old Salvador Ramos, also posted a message saying he was going to shoot his grandmother, as well as another one confirming that he had done so, Abbott said at a news conference. His grandmother, whom Ramos shot in the face shortly before attacking the school, survived and called police. Ramos fled the…