TINKERPOP-1084 Branch option tokens should be allowed to be traversals

[dkuppitz]

https://issues.apache.org/jira/browse/TINKERPOP-1084

Allow predicates and traversals to be used as options in BranchStep.

gremlin> g = TinkerFactory.createModern().traversal()
==>graphtraversalsource[tinkergraph[vertices:6 edges:6], standard]
gremlin> g.V().hasLabel("person").
......1>   project("name", "age", "comments").
......2>     by("name").
......3>     by("age").
......4>     by(branch(values("age")).
......5>          option(any, bothE().count()).
......6>          option(29, constant("almost old")).
......7>          option(__.is(32), constant("looks like josh")).
......8>          option(lt(29), constant("pretty young")).
......9>          option(lt(35), constant("younger than peter")).
.....10>          option(gte(30), constant("pretty old")).fold())
==>[name:marko,age:29,comments:[3,almost old,younger than peter]]
==>[name:vadas,age:27,comments:[1,pretty young,younger than peter]]
==>[name:josh,age:32,comments:[3,looks like josh,younger than peter,pretty old]]
==>[name:peter,age:35,comments:[1,pretty old]]

docker/build.sh -t -i passed.

I will squash the 2 commits before merging.

VOTE +1

[spmallette]

Nice that we didn't have to change the API on GraphTraversal to support this. A couple issues:

1. I think some upgrade docs would be nice in this case, because this is a neat feature to call attention to.
2. I think the test you added is fine, but the problem is that it doesn't validate operations for GLVs. I assume GLVs will work, but it would be nice to see a test added that would validate that.

[spmallette]

VOTE +1