Equations added

This partially addresses issue #321 by adding these equations: ace, enspie, heip_e, fisher_alpha, kempton_taylor_q, mcintosh_e, menhinick, shannon, simpson, and strong. In addition, variable definitions were added to berger_parker_d and to brillouin_d. 
Also, ``with more than 10 individuals`` should be ``with more than `rare_threshold` individuals``. `rare_threshold` is an optional parameter to this function, and it defaults to 10, but users have the option to change this behavior.