Implemented dividers in the NodeEditor.

A divider is introduced underneath a plug when that plug has a "divider" metadata entry of True. RenderMan shader writers may use the annotation "parameterName.divider" with a value of "True", "true", or "1" to enabled this behaviour for a particular shader parameter.

Fixes #288.
Merge pull request #415 from johnhaddon/plugDividers

Implemented dividers in the NodeEditor.