New icons

I spent some time remaking all of the icons. The main reason was that the browser action icon didn't look too great, but I decided to just change every icon since there was (a little) room for improvement. The most important change is that the icons are properly hinted and anti-aliased, so they don't appear blurry any more. This is especially noticeable in the 16px and 19px ones.

The source of the 16px and 19px ones is `ublock-16px.svg`, while all the rest are rasterized versions of `ublock.svg`. The difference is in the white border and the font weight. The small ones have no borders and a bold font; all of the other ones have a white border and a light weight.

The small changes in `async.js` and `pagestore.js` just removed the extra whitespace.