Propose some changes to DataFrames.jl API

[bkamins]

I will comment on the rationale of the changes inline (as I might not understand all the details of the intended design).

[bkamins]

CI is failing because only is not defined in Julia 1.0 but this can be fixed later after we confirm the design intention.

[johnnychen94]

Looks okay to me. Since I don't use DF very much, @oxinabox how does this look to you?

[oxinabox]

Seems right to me though I haven't looked at LearnBase in literally years, and doing #46 seems like a better idea, but this is a net improvement.

[bkamins]

Ah - I have not seen #46. Indeed this PR should be closed if someone is working on #46.