prewitt operator added for edge detection

Prewitt operator added for edge detection.
Read more at: http://en.wikipedia.org/wiki/Prewitt_operator

Example:

    i = Image('lenna')
    p = i.prewitt()
    p.show()



![lenna_prewitt](https://f.cloud.github.com/assets/1975189/405070/4c490fb4-a9af-11e2-82cc-0d000c3ff6c7.png)
Output of applying prewitt operator on lenna

![lenna_sobel](https://f.cloud.github.com/assets/1975189/405071/747c3b14-a9af-11e2-8451-f3c69ff5daa4.png)
Output of applying Sobel operator on lenna