Add Montemurro and Zanette's entropy based keyword extraction algorit…

…hm. Fix #665 (#1738)

* Added Montemurro and Zanette's entropy-based keyword extraction algorithm

* Improved Docstrings

* Fixed numerical bugs due to zero frequencies

* Coding style changes, test and tutorial

* I hate git

* Summarization tutorial

* Fixed some failing tests

* Tests, demo, nan_to_num and a few last flake8 issues

* Further flake8 issues

* Further flake8 issues

* Removed Jupyter checkpoint

* Removed trailing whitespace

* Trailing whitespace

* Speed up test and add comment to explain threshold value

* Flake8 again

* rename vars + style fixes

* fix operation order