extractor: youtube: Swap video dimensions to match standard practice.

While working on this, I thought about simplifying things like changing
480x854 to 480p, and that seemed like a good option, until I realized that
people (me included) usually link the concept of some number followed by a p
with the video being 16:9.

So, we would be losing some information and, as we all know,
[explicit is better than implicit][*].

[*]: http://www.python.org/dev/peps/pep-0020/

This closes #1446.

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>
Merge remote-tracking branch 'rbrito/swap-dimensions'