Average Luminosity Lightness methods implemented.

Methods for Average, Luminosity and Lightness methods implemented.

Example:

    img = Image('lenna')
    i1 = img.getAverage()
![avg](https://f.cloud.github.com/assets/1548906/441558/f03fcda8-b11e-11e2-97ba-142c5c95a824.png)

    i2 = img.getLightness()
![light](https://f.cloud.github.com/assets/1548906/441561/f6dd3862-b11e-11e2-8882-7ce38f50da57.png)

    i3 = img.getLuminosity()
![luminosity](https://f.cloud.github.com/assets/1548906/441562/0307b7d4-b11f-11e2-847a-5bc9536cb3ff.png)


Issue #492