Added an ability to change color of text and background color of widget.

Thats how it looks on my Awesomebox:
![awesoMPD](https://f.cloud.github.com/assets/2613966/321727/05b6df84-9a07-11e2-861d-4b715748d9b6.png)
Usage:
```lua
musicwidget.font_color = "#000000"	--Set font color to white
musicwidget.background = "#FFFFFF"	--Set background color to black
```
By default colors will be taken from beatiful theme. 'fg_normal' for font color and 'bg_normal' for background colour.
This feature should be useful.
If it is I will add the same to the awesome-git branch.