jordan spieth won the 2015 us masters at the augusta national golf club .
spieth won the first major of his career having led all week .
the 21-year-old hit a final round of 70 to finish on 18 under par .
spieth became the first man ever to reach 19 under par in the masters .
the american finished second to bubba watson at the 2014 masters .
spieth : winning the masters has been the most incredible week of my life .