Fix bug. Now the loop ends when fails to converge.
Allow counting of records to work for series as well as dataframes

abundance_in_group was failing when the lack of an abundance
column resulted in trying to use .count() on a Series instead of
a Dataframe. This replaces that call with .size() which works on
both.

It also removes an unnecessary slicing to remove columns that
should never exist.

Fixes #10.
Merge pull request #12 from ethanwhite/master

Allow counting of records to work for series as well as dataframes
Lower tolerance to make quad() in pln more accurate
Multiple tricks to avoid problems for extreme values in pln
Revert to previous version of pln for stability. Change the method in pln_solver() to BFGS for better results.
Add .rvs for pln
Bug fix
Add solver and log-likelihood for Zipf distribution