bright	- adjust brightness of an image

Calling Sequence:

	img = bright(im,rate)
	
Parmeters:

	im: this is a MxNx3 unsigned char matrix(uint8).
	rate: a value between -100 and 100,it shows the degree of brightness
	img: a MxNx3 unsigned char matrix of output image

Description:

	This function adjust brightness of an input image.If "rate" equals to 0,we'll not change the image's brightness.if rate<0,we get an image darker than input.The more ratio,the more brightness.
	
Example:

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