Mapslices to accept non-Number, non-AbstractArray functions

Basically, it is now allowed to use functions which return neither a <:Number nor an AbstractArray.

```
 mapslices(x->(x[1],x[2]), randbool(3,4),2)
3x1 Array{(Bool,Bool),2}:
 (true,false)
 (true,false)
 (false,true)
```

map has this property already:
```
map(x->(x[1],x[1]), randbool(4))
4-element Array{(Bool,Bool),1}:
 (false,false)
 (false,false)
 (true,true)  
 (false,false)
```