Grid method improved 

Bug encountered 

On adding two different Grids, they were getting super imposed instead of getting updated.
Previously it used to give me super imposed grids but now it updates the grid.
    
    img = Image("lenna")
    img.grid().show() 
    img.grid((20,20)).show()

removeGrid() method added.

    img = Image("lenna")
    img = img.grid((20,20))
    img.removeGrid()

It returns the Drawing Layer corresponding to the Grid Layer 

In ShapeContextClassifier even after issue #275 , there was a syntax error