Image Normalization added

Added Image normalization/ contrast stretching.
Read more at:
see : http://en.wikipedia.org/wiki/Normalization_(image_processing)

The results are similar to that of equalization. But this technique does not produce noise. This only stretches the current min and max of the intensity to the specified value.
The noise difference on the edges is completely visible in the logo image.

Example:

    i = Image('lenna')
    a = i.normalize()
    b = i.equalize()

![defnorm](https://f.cloud.github.com/assets/1975189/449318/fca0cc22-b26e-11e2-8525-fdff35f64388.png)
result of normalizatino
![equalize](https://f.cloud.github.com/assets/1975189/449319/fcf6bac4-b26e-11e2-9b5f-ea452241feb2.png)
result of hist equalization
![logo_normalized](https://f.cloud.github.com/assets/1975189/449320/fd0f8748-b26e-11e2-863d-afb34be76709.png)
Normalization of 'logo'
![logo_equalized](https://f.cloud.github.com/assets/1975189/449321/fd2a27c4-b26e-11e2-8d55-f81f94e6cbfc.png)
Equalization of 'logo'