WIP: Consolidate hcat, append, merge, vcat and split functional overlaps

[cjprybol]

removes hcat and hcat!, append(!) now adds columns to end of DataTable, merge(!) concatenates DataTables horizontally, and vcat still concatenates vertically. append! and vcat no longer perform the same function. hcat(!) and merge no longer perform the same function.

Aims to resolve https://github.com/JuliaData/DataTables.jl/issues/52