ENH: Glm add score_obs

so far just adds analytic (glm generic) score_factor and score_obs

see #1775 for score_factor
usage 
#1753 score/LM test
#1738 robust cov

related #1726 generic numerical derivatives in LikelihoodModel

score_factor is the same as score residuals in Stata.

I checked some examples against Stata, but unittests are against discrete and OLS/explicit

add Hessian, observed and expected (information matrix) should be relatively easy to add, but not yet done.  DONE
hackish: problems in varfunc and numdiff  (need to open two issues)
uses numerical derivatives in varfunc and links, should be replaced with analytical derivatives

added score_test to model, not yet to results, maybe it only stays at the model.