Modify create dictionary and new function for corpus

stemming.py: stemming functions for a string and a list of strings which includes everything: stopwords, abbreviations, ngrams, tokenizer and stemming
    At the moment stemmer=English and tokenizer=Punktword, although this can be costumize at any time
create_dic.py: stemming functions have been removed and a new function added: 
                        customizeDic() which loads an existing dicitonary and customize it
bow_matrix.py: New function which loads a dictionary and creates a corpus (bow) from the raw email text based on that dictionary.