Docstrings use references to older pysal bibliography

For example:

```
 
    GMM method for a spatial error model with heteroskedasticity (note: no
    consistency checks, diagnostics or constant added); based on Arraiz
    et al [Arraiz2010]_, following Anselin [Anselin2011]_.

```

We may want to develop a centralized scheme were all pysal projects can reference the same master bib source.