Updated style with more explicit examples of truth checking

[leetrout]

I feel like the existing example if not attr: print 'attr is None!' is very misleading.

Since this is such an opinionated style guide it seems like a great opportunity to link out to the Python documentation on falsey values and give an example of explicitly checking for a value to be None with the is keyword.

[kennethreitz]

Perfect, thanks!

[kennethreitz]

Hmm, I'll have to manually merge this.

[kennethreitz]

Merged!