Add hl-line face.

This pull request adds a face for highlighting the current line. I tried a bunch of colours orange, green, blue, etc.

It seems that the green produces the best results vs. abusing blueish colours, see for yourself below.
- It's easy enough to differentiate between selected text and the current line.
- When no text is selected, it's still easy enough to see clearly all characters.
- When the current line is close to the mode-line, there's also enough contrast.

![screen shot 2014-10-08 at 9 23 46 am](https://cloud.githubusercontent.com/assets/5206067/4559690/7958914c-4eee-11e4-8f8e-929434cce94c.png)

![screen shot 2014-10-08 at 9 24 13 am](https://cloud.githubusercontent.com/assets/5206067/4559692/7fdcab5c-4eee-11e4-8713-fa28af05d270.png)

![screen shot 2014-10-08 at 9 40 26 am](https://cloud.githubusercontent.com/assets/5206067/4559931/d2fc133e-4ef0-11e4-8ec2-13a23df66d7e.png)