jordan spieth wins 2015 us masters after finishing 18-under at augusta .
spieth became the first man ever to reach 19 under par in the masters .
justin rose and phil mickelson -lrb- both 14 under -rrb- finished joint second .
read : spieth has world at his feet after record-breaking masters triumph .
spieth : winning the masters has been the most incredible week of my life .