Handling of DArray input to distributing functions

This is not what you want

``` jl
julia> distribute(distribute(randn(4)))
4-element DistributedArrays.DArray{Float64,1,SubArray{Float64,1,DistributedArrays.DArray{Float64,1,Array{Float64,1}},Tuple{UnitRange{Int64}},false}}:
1.27283
-0.642431
-0.585599
-1.96976
```

The most general solutino might be to change `verified_destination_serializer`. Alternatively, we'll have to add special `DArray` methods for each method defined for `AbstractArray`. @amitmurthy thoughts?

Start by reproducing the nested distribute(randn(4)) example from the issue, then read verified_destination_serializer and the existing DArray methods for AbstractArray operations. Determine whether the fix belongs in the serializer or requires special DArray methods; done means nested distribution no longer produces the unwanted DArray type and the behavior has regression coverage.

索引モデルが issue の本文から書いたものです。