ENTROPY - computes the entropy of a clustering result
    [VENTROPY, CONFUSION_MATRIX, MISTAKES]=ENTROPY(CLUSTERS, 
    LABELS) computes the entropy value of a clustering result 
    represented by the CLUSTERS structure. LABELS is a vector 
    of integers containing the true labeling of the objects. 
    The entropy value is stored in VENTOPY, while 
    CONFUSION_MATRIX is a k x r matrix, where k is the number 
    of clusters and r the number of true classes, and 
    CONFUSION_MATRIX(i, j) records the number of objects 
    of class j assigned to cluster i. Finally, MISTAKES contains 
    the number of misassigned objects, measured by m1+...+mk, 
    where mi=sum(CONFUSION_MATRIX(i, j)), j~=i.
 
  Copyright 2011 Dimitrios Zeimpekis, Eugenia Maria Kontopoulou, 
                 Efstratios Gallopoulos