Adds dependencies for wordcloud generation

There is a problem when installing the wordcloud module if Cython was not
previously installed. If that happens, installing Cython before trying to
install all deps (with `pip install -r`) should solve this.

I forked the original wordcloud module (https://github.com/amueller/word_cloud)
because it needed to be slightly adapted (so it wouldn't show and save the
image, for example).
Adds wordcloud worker
Filters stopwords and punctuation before generating the wordcloud
Updates information about requirements
Documents the need for DejaVuSans
Merge pull request #164 from flavioamieiro/feature/wordcloud

Wordcloud worker