Add threads argument

[rragundez]

Add threads when parsing arguments, add argument to the cli function and add to options sent to gunicorn. Add final dot on some help strings. Remove unnecessary number_of_workers() in options since it is already been set as default in the argument parser. Small change in number_of_workers to make it more Pythonic.

[rragundez]

CN you merge it or close it? What you see fit.