Resolves issue #686.

Updated 3 lines in compare_alpha_diversity.py to check if p_val or obs_t return as None because of low variance data passed to mc_t_two_sample. Updated the script documentation to identify this third source of error (why you would get None, None for pval, tval).

Note: I cannot reproduce the error you are finding @gregcaporaso so I have inspected the code carefully and believe this will resolve the error. If you would like to send me the test data thats failing for you, would be happy to test it.