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`.
Fixes TextCorpus not having proper metadata info.

I've defaulted to returning the line number that a document appears on.
changed `topn` argument to `num_words` 

This brings the `LdaModel.print_topics` and `LdaModel.show_topics` in line with the equivalent `LsiModel` methods.
Merge pull request #1 from piskvorky/develop

catchup develop
'sample' parameter for downsampling frequent-words
simplify subsampling logic
Merge branch 'release-0.10.0' into develop
word2vec: don't call random_sample when sample prob >= 1
Merge pull request #2 from piskvorky/word2vec_sample

Simplify subsampling logic
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...
Merge pull request #206 from gojomo/develop

[word2vec] 'sample' parameter, for frequent-word down-sampling
update CHANGELOG with new pull requests
TextCorpora no longer raises exception on empty input file
Fix length for corpora on empty files
Remove commented out lines
Merge pull request #203 from cscorley/fix_textcorpus_metadata

Fixes TextCorpus not having proper metadata info (rebased)
PEP8 corpora changes
Add description to CHANGELOG.txt
Merge pull request #209 from cscorley/empty_corpora_inputs

Fix length calculation for corpora on empty files