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`.
changed `topn` argument to `num_words` 

This brings the `LdaModel.print_topics` and `LdaModel.show_topics` in line with the equivalent `LsiModel` methods.
Merge remote-tracking branch 'upstream/develop' into develop
Merge pull request #201 from Tiepies/develop

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