Space after all commas

[cc7768]

The Python cheatsheet has several places where there aren't spaces after commas (I noticed while I was reading some of the notifications this morning):

• https://github.com/QuantEcon/QuantEcon.cheatsheet/blob/master/python-cheatsheet.rst#iterating
• https://github.com/QuantEcon/QuantEcon.cheatsheet/blob/master/python-cheatsheet.rst#user-defined-functions
• https://github.com/QuantEcon/QuantEcon.cheatsheet/blob/master/python-cheatsheet.rst#numpy
• And others...

We should have spaces after all commas in both Python and Julia (I didn't notice any missing spaces in Julia but it wouldn't hurt to check).

The fact that our team is growing also probably means that we should move the style guide out of our heads and onto paper at some point -- We started a Julia version awhile back.

[jstac]

+1 for spaces after commas

[rhozacc]

#47 This should fix it.