Appends exceptions to a list instead of overwriting the previous value
Saves worker name as well as the traceback for exceptions

I have decided to keep old exceptions as simple strings, but it should not be
common to have this in a real database, since documents are rarely reanalyzed.

This and the previous commit should close #162
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
Merge pull request #163 from flavioamieiro/fix/exception_logging

Append (instead of overwrite) exception logging

Fix #162
Documents the need for DejaVuSans
Merge pull request #164 from flavioamieiro/feature/wordcloud

Wordcloud worker