Implement generic csv import

[aboodman]

A lot of data in the world is csv or tsv. It would be good to have generic support for this.

An interesting question is how to represent csv in noms types. Assuming the column names are known, the first thing that comes to mind is a list of maps. But that would have a problem: it wouldn't enforce that all the maps are the same shape, so it's hard to do things like graphs.

What we really want is List<T> where T is a struct with fields corresponding to to the columns of the csv.

We can easily create a StructDef dynamically using the column names as the fields. But it is not easy with the current generated Go code to dynamically create instances of such a struct. Perhaps there should be.

[aboodman]

See also issues #20 and #21

[cmasone-attic]

Closed as dup of #29