Fixed TypeError and standardized `num_topics` argument in `LdaModel.show_topics`

Line 678 of gensim/models/ldamodel.py threw a TypeError under Python3+, because the single slash division syntax returns floats, which cannot be used as indices. I changed this line to use floor division instead. This does not break Python2 compatibility.

Also solved Issue #115 by changing the `topics` argument of `LdaModel.show_topics` method to `num_topics`.