re-factor code

[pdeffebach]

Now that the argument-handling code is so long, it's kind of hard to work with the big DataFramesMeta.jl file.

Is re-factoring into separate files always discouraged? Or is there a way to do it that preserves line-by-line commit histories.

cc @nalimilan

[bkamins]

This is a good idea. I propose you to just split the code into files but without changing anything (then I can easily check that all is OK). And such a PR should contain only "code movement"

[pdeffebach]

done in #243

[bkamins]

OK to close?