ATHENS (Reuters) - The first authoritative opinion poll published since Greek Prime Minister Alexis Tsipras resigned last week showed his leftist Syriza party leading the opposition conservatives ahead of Sept. 20 elections, the Efimerida Ton Syntakton newspaper reported on Friday. The Syriza party was supported by 23 percent of those polled, versus 19.5 percent backing the conservative New Democracy party, according to the poll, which was carried out by ProRata based on a sample of 1,000 interviewees. ...