Dispersion weighting

Hai Jari,

in a discussion with Bob Clarke he pointed me to one of his papers:

>Clarke, K. R., M. G. Chapman, P. J. Somerfield, and H. R. Needham. 2006. Dispersion-based Weighting of Species Counts in Assemblage Analyses. Marine Ecology Progress Series, 320, 11–27.

As far as I know this hasn't been implemented in R yet, so I wrote it up myself. 
This may fit very well into the scope of vegan, therefore the pull request.

Just let me know if I should change anything (eg the name if the function, coding style, etc.). The function isn't optimized yet, and I think there is some room for improvements....


Kind regards,

Eduard