[API]: Multivariate Kendall tau and Spearman rho.

[lrnv]

According to what we found in #133, we should modify the current implementation of \tau and \rho so that they return (d,d) matrices on d-variate copulas, containing bivariate kendall and spearman coefficients. This will match correctly what is outputed by Statsbase.corkendall and Statsbase.corspearman. The fitting of archimedeans needs to be re-written a bit to take this into account..

Then the API could be exported.