Plot residuals of a model against fitted values, a QQplot of residuals and perform a Shapiro-Wilk test on residuals. The function deals with lm (including glm and glm.nb) and mer models. Quantile residuals (qresiduals() function, statmod package) are used for glm.nb models and for glm models with a poisson, quasipoisson, binomial or quasibinomial family.