By Steve Scherer and David Ljunggren OTTAWA (Reuters) -Canadian Prime Minister Justin Trudeau plans to invoke rarely used special measures allowing him to tackle protests that have shut some border crossings and paralyzed downtown Ottawa, sources said on Monday. In the Western Canadian province of Alberta on Monday, police broke up an armed group that was prepared to use violence to back a blockade at a border crossing with the United States, authorities said. Trudeau plans to use the 1988 Emergencies Act, which allows the federal government to override the provinces and authorize special temp…