Added GafferImage::ImageWriter::acceptsInput() method.

Implements an acceptsInput() method on the ImageWriter node. The method checks ExecutableNode::acceptsRequirementsInput() to ensure that an ImagePlug cannot be connected to a child of the "requirements" plug.

This solves the issue outlined in #681 but a more elegant solution should be found by moving the logic into the base class.

This should also address #255.