Nevada's top election official has filed a civil lawsuit against Americans for Prosperity, alleging the Republican conservative group violated state election laws by not registering in the state or filing donor contribution reports. The suit filed in state court in Carson City late Friday by Secretary of State Ross Miller claims the group founded by billionaire brothers Charles and David Koch engaged in "express advocacy" against Kelvin Atkinson, a state Senate candidate who won both the Democratic primary in June and the November general election.In a statement Saturday, Adam Stryker, AFP state director, said the complaint filed by the attorney …