Add default args in docstring and Format code
Add default args in docstring and Format code

I have included optional default arguments in the docstring of all
functions.

All trailing whitespaces are now fixed conforming to PEP8. The check is
done via pycodestyle and http://pep8online.com/.

QuantEcon/QuantEcon.py,QuantEcon/QuantEcon.py,oyamad/QuantEcon.py,oyamad/QuantEcon.py