Handle F# Data dependency (for CSV parsing) via Github

This removes the dependency on FSharp.Data package. We can now reference the files we need directly from F# Data Github using a new nifty Paket feature.

The benefit is that we have a smaller stand-alone package that can be combined with any version of F# Data and also is less prone to binding redirect errors (no MethodMissingException when using wrong FSharp.Core.dll).

The only slightly tricky thing is that we need to find a closed set of files from F# Data which covers all the functionality we need. This includes the type inference & CSV parsing, but there are other helpers (e.g. for IO). Since I did not want to include _all_ files, just because of a few unused helpers, I included `Stubs.fs` with three functions that I did not want to copy from F# Data.

[This adds more things on top of #288, so the diff will be nicer after that one is merged.]
Will these files appear as "public" in Deedle, under the FSharp.Data namespace (the same as in FSharp.Data itself)?