Add solution for Expert 4

Does this qualify as "at once" ?

I had wanted to use `@devec` to do this without the temporary, but for the moment (as far as I can make out) this appears not to be possible https://github.com/lindahua/Devectorize.jl/issues/31.

Also, fix question 3 (atm it's a repeat of 2).
In order to return same result as [numpy version](http://www.loria.fr/~rougier/teaching/numpy.100/#expert), 
could you set  `p, n = 10, 20` and use `ones()` instead of `rand()`?

I cite numpy version answer FYI

```python
p, n = 10, 20
M = np.ones((p,n,n))
V = np.ones((p,n,1))
S = np.tensordot(M, V, axes=[[0, 2], [0, 1]])
print S
# returns 
#[[ 15.]
# [ 15.]
# [ 15.]
# [ 15.]
# [ 15.]]
```