Tweaked the icon a bit

This changes the meteor icon by removing the black background and changing the foreground from white to 50% gray. This looks great under both dark and light UI theme variants.
![screenshot at 03-39-41](https://cloud.githubusercontent.com/assets/138886/3787533/93f86dae-1a20-11e4-8b32-3f5e4ab3ed7e.png)
![screenshot at 03-40-27](https://cloud.githubusercontent.com/assets/138886/3787535/ab550d36-1a20-11e4-963b-81b79d4829e3.png)