broadcast issue

Hi,

In order to have AF working alongside with CuArrays, I would need the following to work:

```
a .= a .+ b
```
but it returns

```
julia> a .= a .+ b
ERROR: MethodError: no method matching ndims(::Type{AFArray{Float64,1}})
Closest candidates are:
ndims(::Base.Generator) at generator.jl:53
ndims(::Number) at number.jl:67
ndims(::Type{#s75} where #s75<:Number) at number.jl:68
...
Stacktrace:
[1] Base.Broadcast.BroadcastStyle(::Type{AFArray{Float64,1}}) at ./broadcast.jl:103
[2] combine_styles(::AFArray{Float64,1}) at ./broadcast.jl:405
[3] Base.Broadcast.Broadcasted(::typeof(identity), ::Tuple{AFArray{Float64,1}}, ::Tuple{Base.OneTo{Int64}}) at ./broadcast.jl:175
[4] materialize!(::AFArray{Float64,1}, ::AFArray{Float64,1}) at ./broadcast.jl:804
[5] top-level scope at REPL[7]:1
```

Is it an easy fix?

Btw you should consider making AF a backend for `GPUArrays`

Thank you,

Best