channelMixer added

Channel Mixer added.
Mixes channel of an RGB image based on the weights provided. The output is given at the channel provided in the parameters. Basically alters the value of one channelg of an RGB image based in the values of other channels and itself. If the image is not RGB then first converts the image to RGB and then mixes channel

Definition:

    channelMixer(self, channel='r/g/b', weight= (a,b,c))

Examples:

    img = Image("lenna")
    img2 = img.channelMixer()