JuliaStats/DataFrames.jl — pull request #726 by vchuravy (2014-11-17 13:28:40+00:00)
Title: [WIP] Iterate over a subset of cloumns with colwise

I often find myself writing code like this

````julia
using DataFrames, RDatasets
iris = dataset("datasets", "iris")

by(iris, :Species) do subdf
means = colwise(mean, subdf[cols])
DataFrame(means, cols)
end