Merge pull request #11 from lindahua/master

Expanding logsumexp, softmax, and pventropy to allow per-column and per-row computation
mad(v): only make one array copy and use it as scratch space.
mad(v): avoid checking for NaNs when taking the median of deviations.
Merge pull request #13 from StefanKarpinski/master

median absolute deviation performance improvement
add sample_by_weights (tested)