rPref 0.7
=========

* New page "general_pref" in the documentation collecting
  some useful functions for preference objects

* Moved length, as.expression, as.character from show_pref to
  to general_pref
  
* Moved empty() from complex_pref to general_pref
  
* New functions is.preference, is.complex_pref, is.base_pref, 
  is.empty_pref (in general_pref)

* Fixed visibility of as.expression() function for preference objects

* Changed behavior of show_level parameter of the psel function

* Parameter "dialect" in show.query is not case sensitive anymore

