Create Serializers for Graph objects in Gremlin-Python

Right now, Gremlin-Python only supports VertexDeserializer, etc. We should add VertexSerializer. Why? So we can send vertex, edge, etc. objects in a traversal.

v = g.V().has('name','stephen').next()
g.V(v).out('knows').toList() // currently doesn't work
g.withSideEffect('x',v).V().out('knows').where(neq('x')) // currently doesn't work

Note that the serialization simply needs to encode id only so this can be really simple to do.