Added WeightStatsMonitor as a copy of WeightSumMonitor

Made appropriate changes that were required. Should close issue #1 if
I've done everything right.
Merge pull request #3 from sanjayankur31/weightstatsmonitor

Added WeightStatsMonitor as a copy of WeightSumMonitor. This is the first step to leviate the problem of WeightSumMonitor currently not computing the actual sum of any weights.