Add alpha texture image

And addition of some missing doc.
Now by using : 
viewer.setFillTransparency(DGtal::Color(255, 255, 255, 120));
we will obtain a display with transparency:
![capture decran 2013-06-07 a 12 27 27](https://f.cloud.github.com/assets/772865/623227/285b8b8a-cf5d-11e2-879b-8bec20aeda0c.png)