Fixed TypeError and standardized `num_topics` and 'num_words' arguments in LdaMallet 'show_topics' and 'print_topics'

(This fix is the LdaMallet equivalent to #201, which fixed LdaModel)

Added Python 3+ integer division (`//`) to `show_topics` in LdaMallet and renamed the arguments `'topics' to `num_topics` and `topn` to `num_words` to keep LdaMallet consistent with LdaModel.  Does not break compatibility.