Replace DataFrames with Tables

[oxinabox]

So we can work with all types that implement the Tables.jl API.

I think this will solve #39 as a side-effect, or at least will be solid progress towards that

[ericphanson]

This would also fix the compat issue of https://github.com/JuliaML/MLDataUtils.jl/pull/59

[ppalmes]

is there any fix?

[oxinabox]

No. @JuliaRegistrator register