Make setindex! return the updated DataArray

For consistency with `Base`:
```julia
v = [1:7]
r = setindex!(v, 4:6, 1:3)
r
7-element Array{Int64,1}:
 4
 5
 6
 4
 5
 6
 7
```
~~And I want to get rid of some unnecessary allocation in `DataFrames`' `setindex!` methods, but doing so would be inconsistent with `DataArrays` though consistent with `Base`.~~