Update flash icon to a lightning bolt.

This is a pull request for #33. I work with @mkcode.

I tried to maintain the exact same dimensions of the icon images, as well as the same selected color.

This is what the icon looks like.
![slice 1](https://cloud.githubusercontent.com/assets/171215/3059426/552a4614-e1eb-11e3-9ed8-67e1ddd5e2a9.png)