Add PAST permutation tests.

[TGuillerme]

As used in Brusatte et al. (2014, Current Biology, 24, 2386-):

we used permutation tests to perform pairwise comparisons of group means in R and PAST. These comparisons tested the equality of multivariate means (based on the 125 recovered PCO axes, which comprise 90% of total variance) of two designated groups (for example, avialans versus deinonychosaurs). The Mahalanobis distance between the two group means was calculated and compared with a null distribution of between-group distances obtained by random permutation of the group labels.