Clean up toolbar images and add Retina assets.

@brendandahl asked about high-DPI (retina) assets. This adds those and does some clean-up on the existing icons flattening them out a bit and bumping up the contrast.

![pdfjs-images](https://f.cloud.github.com/assets/684427/2223683/ba77d94a-9a75-11e3-91d1-4000c077f3e2.png)