Fix sizes attribute to be compliant with the spec.

Actually `a_sizes [40; 50]` will output sizes="40 50" which seems to be wrong(IMHO) because as it says in [HTML5 spec](http://www.w3.org/html/wg/drafts/html/master/links.html#attr-link-sizes), `sizes` is a space-separated list of size(widthxheight).
Also, the spec says `sizes` value can be `any`.

This pull request fix those things, `a_sizes` takes now `` `Sizes [(480, 60); (500, 500)]`` (e.g) or `` `Any``.

Note : As it seems that [sizes](http://www.w3schools.com/tags/att_link_sizes.asp) attribute is currently not implemented in any browser, merging this pull request is not really urgent.