ENH : add support for 2 samples spatio_temporal clustering
ENH : uniform API of permutation_cluster_test and permutation_cluster_1samp_test adding step down etc. to permutation_cluster_test
ENH : refactor to merge permutation_cluster_1_samp_test and permutation_cluster_test
ENH : adding default thresh in permutation_cluster_test
extra space
Merge pull request #506 from agramfort/spatio_temporal_cluster_2samp

ENH : add support for 2 samples spatio_temporal clustering