Custom attributes/options

Addresses #272 - adds two new nodes, CustomAttributes and CustomOptions

- two new nodes derive from Attributes and Options respectively
- added to the node menu
- updated the UI files for the old nodes to attempt to clarify that they are a base class
- moved the detailed usage notes to the new UI files
 - note: I don't think the nodeDescriptions are picking up properly. Not sure if that's me or gaffer. I note that the NodeEditor lists the nodes as being of the base type... so maybe i've not quite got that right