High-res arrow icons

I added in high-res arrow icons so they look better on Retina displays and the like. Additionally, if the information my computer gave me is correct, all the current arrow images take up 5,893 bytes, but these new images only take up 705 bytes total; also, there are currently three arrow images, whereas if we merge this there will only be two. (Also, the "tools" arrow is flipped around to match the two on the top bar.)

Here's what the arrows look like currently on a Retina display:
![normalres](https://f.cloud.github.com/assets/5515877/1770876/aedec560-67a0-11e3-84ac-1678737b2910.png)

Here's what they look like with these commits:
![highres](https://f.cloud.github.com/assets/5515877/1770871/6c9fa016-67a0-11e3-8661-f5f0c047696c.png)