setindex! should return the array, not the value

Eg.

```julia
julia> setindex!(ones(3),2,1)
3-element Array{Float64,1}:
 2.0
 1.0
 1.0

julia> setindex!(OffsetArray(ones(3),1:3),2,1)
2
```

It might be better if `setindex!` returned the array to be consistent with `Base`. 
