Updates relating to issue https://github.com/statsmodels/statsmodels/issues/630

I have attempted to address the problems pointed out in the issue here:
statsmodels#630

I have addressed:
-- pylint finds 3 undefined variables
-- plot_influence and plot_leverage_resid2 are missing from docs AFAICS
-- add example plots to docstrings (have only added one plot for now, if this is okay I will add more).

I cannot see that there are any functions to switch to xxx_grid functions for the comment
-- "statsmodels\statsmodels\examples\ex_regressionplots.py" needs updating
mainly switch to xxx_grid functions

Note: The function "plot_influence" does not exist, instead being called "influence_plot". Perhaps this should be changed for consistency?

(This is my first commit, so go easy!)