BUG Copy result of ndarray.diagonal()
BUG Copy result of ndarray.diagonal()

In newer versions of numpy (after 1.9), this is a view and, thus,
changes, when the array is modified. In previous versions, it was a
copy.

Closes #16

pombredanne/milk,luispedro/milk,pombredanne/milk,luispedro/milk,luispedro/milk,pombredanne/milk