Added two themes

Hey @bling,

Added these two themes, they are simple and subtle. Background color is same (irrespective of mode) on both of them, only the color of the foreground (file name, mode, ... ) changes

- Raven (for Dark colorschemes)

![raven](https://cloud.githubusercontent.com/assets/1803997/4022900/4638ed46-2b64-11e4-8c07-c5ec5620bff2.png)

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- Silver (for Light colorschemes)

![silver](https://cloud.githubusercontent.com/assets/1803997/4022901/4c369aea-2b64-11e4-8731-ac4a229bdef9.png)

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P.S. I had these themes made much earlier (#485), and was lazy to send this PR, Sorry about that ! :smile: