add a NGramme class to represent uni-grammes, bi-grammes, ..., n-grammes
precise documentation in BayesAssigner class
modify BayesAssigner to not take in charge words but n-grammes
add a getter to access the degree of the n-gramme
add equals() and hashCode() methods
modify FrequencyAssigner to take n-grammes in charge
modify PresenceAssigner to take n-grammes in charge
Merge branch 'test' into develop
REFACTORING: assigners are now classifiers