v0.2-2

New features

• Add AIC.pltree to enable computation of AIC on new observations (e.g.
  data held out in cross-validation).
• Add fitted.pltree to return combined fitted probabilities for each choice within each ranking, for each node in a Plackett-Luce tree.

Bug fixes

• vcov.PlackettLuce now works for models with non-integer weights (fixes #25).
• plot.pltree now works for worth = TRUE with psychotree version 0.15-2 (currently pre-release on https://r-forge.r-project.org/R/?group_id=330)
• PlackettLuce and plfit now work when start argument is set.
• itempar.PlackettLuce now works with alias = FALSE