##Java 7, 86 bytes

No fancy folds or matrices, just a good ol' fashioned `for` loop :)

    int[]o(int[]a,int l){int i=0,o[]=new int[l];for(;i<a.length;)o[i%l]+=a[i++];return o;}

[Try it on Ideone][1]

Lined:

    int[]o(int[]a,int l){
        int i=0,
            o[]=new int[l];
        for(;i<a.length;)
            o[i%l]+=a[i++];
        return o;
    }


  [1]: https://ideone.com/j3MjBh