predint 2.0.0

• the package is now based on s3 objects
• the calibration algorithm works now independently for both limits
  (default for discrete intervals)
• rqpois() can be used with offsets and returns a data.frame (instead of a
  vector as before)
• quasi_pois_pi() works now with offsets and based on the modified calibration algorithm
• new functions
  • summary.predint
  • plot.predint
  • as.data.frame.predint
  • print.predint