Refine names!, rename!, and rename for AbstractDataFrame
Refine names!, rename!, and rename for AbstractDataFrame

They now return an AbstractDataFrame rather than its Index.
rename and rename! for dfs can use all methods defined for Index.

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