H = hadamard(n)
Hadamard matrices are matrices of 1's and -1's whose columns are orthogonal,
H'*H = n*I
where [n n]=size(H) and I = eye(n,n) ,.
An n-by-n Hadamard matrix with n > 2 exists only if rem(n,4) = 0. This function handles only the cases where n, n/12, or n/20 is a power of 2.
 Ryser, H. J., Combinatorial Mathematics, John Wiley and Sons, 1963.
 Pratt, W. K., Digital Signal Processing, John Wiley and Sons, 1978.