contrast - adjust contrast of an image

Calling Sequence:

	img = contrast(im,rate)
	
Parmeters:

	im: matrix of an input image
	rate: a value between -100 and 100,it shows the degree of contrast
	img: output image matrix

Description:
	This function adjust contrast of an input image.It relates to the diffence between bright and dark.
        The diffence is clearer if "rate" is larger.

Example:

	im = imread("lena.png");
	img = contrast(im,25);