map with multiple arguments

It would be great if map could also be called with 2 or more distributed arrays as argument. Currently the fall-back map is used.

```julia
@everywhere using DistributedArrays
x = distribute([1:10;])
y = distribute(2*[1:10;],x);
@which map(x -> x+1,x)
@which map(x,y -> x+1,x,y)
```

The output is the following:
map(f, d::DistributedArrays.DArray) at /home/abarth/.julia/v0.5/DistributedArrays/src/mapreduce.jl:6
map(f, iters...) at abstractarray.jl:1711

In any case, thank you for your great library!

Start with src/mapreduce.jl, especially the map implementation reported in the issue, and reproduce the two @which calls using the supplied DistributedArrays example. Trace how map dispatch handles one distributed array versus multiple iterators. Done means the multi-array call no longer falls back to abstractarray.jl and has coverage for the demonstrated case.

Scritto dal modello di indicizzazione a partire dal testo della issue.