Merge pull request #3309 from juliantaylor/optimize-byteswap

ENH: optimize byteswapping via intrinsics
BUG: Correctly pass on ddof paramter on inside np.ma.corrcoef

While ddof has basically no effect on corrcoef, it exists,
but it was not passed on correctly (instead only bias would
be passed on). Fixes gh-3336
Merge pull request #3337 from seberg/issue-3336

BUG: Correctly pass on ddof paramter on inside np.ma.corrcoef
add benchmark script
vectorize sqrt on amd64
vectorize simple arithmetic operations
vectorize add and multiply reduce operations
prelimary minmax vector, no nan support yet
add nan support tests and some other stuff
remove a gcc __attribute__
add utility functions for loop blocking
vectorize int16 byteswaps

larger bswap vectorization requires ssse3
add some byteswap test