Add CCEffectSaturation

The shader computes the grayscale version of the image and lerps between
this and the original using the saturation value.
Remove trailing whitespace
Add multiple pixel sizes to the pixellation test
Change CCEffectPixellate to use a pixel size parameter instead of pixel scale

This addresses Viktor's feedback in issue #670 that pixel scale was unclear (Which it was
especially since it wasn't even correctly named. It was really the uv step value not a scale
factor at all). Now the user can specify the size of the pixel blocks and the effect does the
math to convert this into uv step values.
Add GPUImage's license text in the header of those effects for which GPUImage is used as a reference
Merge remote-tracking branch 'origin/develop' into develop
Remove trailing whitespace
Condition input pixel size values

Clamp values less than one to one.
Add the option of adding no effects to the effectNodeWithEffects helper
Change pixelSize to blockSize

And add init and effectWithBlockSize methods.
Add init and effectWithSaturation
Add saturation values outside the range 0..1 to demonstrate this effect
Add init and effectWithBrightness
Add init and effectWithContrast
Merge remote-tracking branch 'origin/develop' into develop
Change "pixel size" to "block size" in the pong demo