augment.lm for missing observations

Old version of augment.lm will stop with error when there are missing observations in the original data.frame.
This is due to different number of rows in original data.frame and x$model. This pull request corrects this error.

Try doing the following:

cars_na <- cars
cars_na$speed[7] <- NA

model <- lm(data=cars_na,dist~speed)
augment(model,cars_na)

A great idea of a package :)