The pilot reported that the windsock was indicating the wind was from 290 to 300 degrees in excess of 15 knots when he landed on runway 21. He stated that the airplane encountered windshear as it approached the runway. The shift in the wind and the increase in speed resulted in the airplane floating down the runway prior to touching down. The pilot reported he was unable to stop the airplane with the brakes and it traveled off the end of the runway where it nosed over upon encountering an embankment. He elected not to perform a go-around due to powerlines near the departure end of the runway. The empennage and wings were substantially damaged. Wind conditions reported 23 miles from the accident airport were 020 degrees at 16 knots, gusting to 22 knots.