New theme: Zadia

I've created a new theme based on the official one, with the aim of making the interface simpler, cleaner and more user friendly.

It has a new color scheme, some new icons (brand new and based on copyleft ones, credited on the theme), and some visual adjustments. As the HTML structure is the same it does not have many changes in that sense, but the GUI has been flattened to accomplish a simpler and easier to the eye look.

![image](https://cloud.githubusercontent.com/assets/240085/3398174/986dc4a4-fd2b-11e3-8c4e-bb7b1a657c10.png)

In the image attached you can check the theme, although the committed and pushed theme uses the default kimai logo, not zadia's one.