Watershed method implemented

Implemented the watershed method 

Basic technique used is, first generate the marker by extracting the foreground and the background and then use the marker and the original image as input to the watershed method of OpenCV.

Usage:

    img = Image("../sampleimages/wshed.jpg")
    img1 = img.watershed() #GrayScale Output
    img1.show()
    img2  = img.watershed(color=True) # colored output
    img2.show()

![wshed](https://f.cloud.github.com/assets/1548906/164949/e6f36286-7902-11e2-8915-7f79b8457362.jpg)

![test_watershed1](https://f.cloud.github.com/assets/1548906/164950/fedd3106-7902-11e2-877a-b3ab57b89417.jpg)