Fixed Sampler indentation.
Added missing default value to GafferImage::Sampler python constructor.
Added ImageSampler node to GafferImage.

This takes an input image and pixel position, and outputs the colour at that pixel.
Fixed comment in ImageView.
Fixed colour space used for ImageView colour sampling.

It was sampling colours after gamma, exposure and luts were applied. Now it samples them before while the image is in linear space. This is a first step in refactoring the ImageView so that the image stats and under-the-mouse samples are computed by nodes and displayed by PlugValueWidgets in an overlay - all the computation is in nodes now but the display is still using miscellaneous OpenGL calls.
Merge pull request #691 from johnhaddon/colorSampling

Color sampling