Retinify

I added Retina support along with some rethinking of how the rounded rectangle is drawn. Now it uses a curved path with no stroke, only fill, and the corner radius is adjustable from a local variable - it would be easy to govern it from the options now. I hardcoded the corner radius to be 3, that gave me the closest result to the original version on low-res.

![screen shot 2013-09-07 at 6 11 46 am](https://f.cloud.github.com/assets/3217744/1100699/88550e3e-177a-11e3-9ec3-67ca6689948c.png)
On the image the leftmost tab shows the original version, the second one shows the current version on low resolution, and the rightmost one shows the new version on high resolution.

The favicon itself will be high resolution if a 32x32 icon is present, otherwise we fall back to the 16x16 version.