Picture article

Hi!

@mathiasbynens, @andreasbovens, @brucelawson & @zcorpan
As discussed (and fashionably late, sorry about that) here's the article I wrote about the picture element.

Cheers :)
Yoav

Are you saying the source version is in Markdown? If so, would you mind submitting that one instead of the HTML version? ([All articles on Dev.Opera are Markdown-formatted](https://github.com/operasoftware/devopera/tree/master/articles/_posts) so it would save some time on our end.)