Updated screenshots to show new UI

Made some new screenshots to show the new user interface as of version 0.5. I also increased the size of the "small" screenshot to be the width of the column GitHub displays README.md in. Both screenshots are compressed as much as possible for their quality & size.

Screenshot of new size:
![image](https://f.cloud.github.com/assets/198225/178614/2d09743e-7bc1-11e2-999e-a3de07091290.png)