Added unit tests for node classes.

@danieldahan Some initial unit tests ready for review.

Note, the following minor issues have been fond in the SDK:
- Duplicate definition of convenience init(type: String!, identifier: String!) in SHAction and SHBond, while it is already defined in the superclass.
- toJSON should be renamed to dictionaryRepresentation
- nodeType is not serialized to JSON
This should be bought.subject!.toJSON(), so change the "?" to "!"