Known bug:

1. calibrate and postStratify do not recompute the
   finite population correction. This matters only
   when the sampling fraction is very close to one

2. svydesign should check that `weights` is the right length

3. options(survey.lonely.psu="adjust") does not do what it says in the 
case of svytotal(). Not yet clear whether it does the right thing, though.
