Add skbio.maths.diversity.alpha module and tests

Currently only has observed_species. Also updated README with names of original authors of this code.
Add osd
Add berger_parker_d
Add brillouin_d
Add dominance
Add singles and doubles
Add enspie, simpson, and simpson_reciprocal
Add mcintosh_d and mcintosh_e
Add simpson_e
Add strong
Add shannon
Add equitability
Add menhinick
remove useless docstrings
Add robbins
Add margalef
Add kempton_taylor_q
Add heip_e
Adding to_file function to OrdinationResults
Fixing tests