Check for columns that are all NA's
Check for columns that are all NA's

emilbayes/DataFrames.jl,scls19fr/DataFrames.jl,tshort/DataFrames.jl,JuliaPackageMirrors/DataFrames.jl,maximerischard/DataFrames.jl,ccphillippi/DataFrames.jl,pwoolcoc/DataFrames.jl,zhmz90/DataFrames.jl,niloynibhochaudhury/DataFrames.jl,maximerischard/DataFrames.jl,JuliaStats/DataFrames.jl,anriseth/DataFrames.jl,campanja-forks/DataFrames.jl,tkelman/DataFrames.jl,armgong/DataFrames.jl