Darkly: Flatly based dark swatch

Hi Thomas,

I needed something like Flatly from @jenil27, but a dark variant so this is how Darkly was born (well, inherited from Flatly). It has a few slight modifications though, such as headings are serif instead of sans-serif, IMHO it looks more elegant on the dark background.

- Do you see it worth merging back to your repository?
- If yes, what do you see as points for improvement?

P.S. `thumbnail.png` is from Flatly; I will update it as we discuss.

Here is a screenshot:
![image](https://f.cloud.github.com/assets/4745368/2283275/f69ca8f2-9fba-11e3-95ba-65f94c24c9f8.png)