Print distributed matrices

We'll have to figure out how to print a `DistMatrix` without getting `nworkers()` times the matrix in the output. Branching such that only rank zero is printed hangs.

``` jl
julia> using MPI

julia> man = MPIManager(np = 4);

julia> addprocs(man);

julia> using Elemental

julia> @mpi_do man A = Elemental.DistMatrix(Float64)

julia> @mpi_do man Elemental.gaussian!(A, 10, 10)

julia> @mpi_do man if MPI.Comm_rank(MPI.COMM_WORLD) == 0; println(A); end # hangs
```