Permuted OLS: Correct the permutation scheme in haxby example.

The mean image per session and per condition is now considered.
Indeed, if no distinction is made, the observations are not exchangeable
across sessions and the permutation scheme is therefore wrong.
Permuted OLS: Address bthirion's comments.
Merge pull request #189 from VirgileFritsch/clever_permutations

[ENH] Permuted OLS: Correct the permutation scheme in haxby example.