DTM wrapper bug fixes.

[bhargavvader]

Changed topn for num_words, wrt #769 .

While the same problem as in LdaMallet wrapper (PR #767 ) for it not being backward compatible is there, the PR #755 already made the non-backward compatible changes so I don't think this should be a problem. (@piskvorky , please have a look at this)

At any rate, either only topn or num_words should be there (not half and half), the print_topics function will not work otherwise.

I've also removed six.u with respect to #768 as well.

Edit: Made changes to the DTM notebook to reflect this as well.

[bhargavvader]

@piskvorky , @tmylk , could you review?

[piskvorky]

I like topn (or perhaps top_n) a little better, because it's not dependent on what the features represent (words, phrases, entities, characters...).

But it's not a big deal either way. We should just be consistent throughout, whichever terminology we use.