Addresses #272 - adds two new nodes, CustomAttributes and CustomOptions

- two new nodes derive from Attributes and Options respectively
- 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
Replacing old 'Attributes' and 'Options', with the new CustomVariants
fixing typo
replacing the python node definitions with c++ ones.
updates the test cases for new Custom* nodes
updates the test cases for new Custom* nodes
fixes missing prefix
Merge pull request #523 from bentoogood/customAttributesOptions

Custom attributes/options