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?