Allow the GremlinGroovy AnnotationHandler to deal with non-iterable
return types. Previously, if a non-iterable type was used as the
annotated method's return type the result would be a ClassCastError.

Like the Adjacency annotation, if there is more then one item returned
from the traversal the remainder will be ignored.
Merge pull request #18 from mikesname/gremlin_single_items

Allow the GremlinGroovy AnnotationHandler to deal with non-iterable return types.