Changes in Version 2.7.4 (February 2017)
  o the observed Fisher information matrix is now returned 
    as attribute ('FisherI') of the returned object

Changes in Version 2.7.3 (January 2017)
  o New baseline hazard family: inverse Weibull (Fréchet)

Changes in Version 2.7 (January 2017)
  o optimization is now based on the optimx package
  o the default optimizer is now nlminb (from the stats package)
  o fixed issue in the likelihood with lognormal frailties
  o new function coeff()


Changes in Version 2.6.0 (November 2016)
  o new baseline distribution: log-skew normal


Changes in Version 2.5.10 (September 2015)
  o added Kendall's tau for the lognormal frailty distrubution


Changes in Version 2.5 (October 2012)
  o new frailty distrubution available: lognormal


Changes in Version 2.01 (June 2012)
  o Added function anova.parfm() to compare models


Changes in Version 1.01 (March 2012)
  o Added stratification


Changes in Version 0.66-2 (February 2012)
  o Added example datasets
