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. 
maybe use N here instead of Q for total individuals, and S for number of OTUs?

and maybe "... the expression in brackets is computed for all OTUs i, and :math:`max_i` denotes the maximum value in brackets for any OTU."

Though I don't love that wording either. Talking in math is hard.