REPL show of DArray makes 4 round trip calls for each element.

I noticed that the default `show` of `DArray` (i.e., it defaults to the one in Base) results in 4 round trip calls for each element. We ought to define ` Base.show(io::IO, mime::MIME"text/plain", D::DArray)` in this package.

The question is should it
1) Only print DArray meta data (which worker has which indexes and so on) or
2) Efficiently call a `convert(Array, D)` and then print it in one go locally or
3) Pull in and show only the first few and last few elements of the darray.