vcov()/SE calculation straight from Hessian

when possible (add use.hessian argument to vcov and summary)
closes #47
version bump
correct lmer objective function to fully account for the influence of prior weights
prior weights tests and commenting
prior weights documentation
update NEWS
minor NEWS edit
update d_ldW in setWeights
remove old debugging code
Merge branch 'master' of github.com:lme4/lme4

merge