Traverser tags as a safer way of using path labels

We should add the following methods to Traverser.

void Traverser.addTag(final String tag)
boolean Traverser.hasTag(final String tag)
void Traverser.dropTag(final String tag)
void Traverser.dropTags()

What is the point of a "tag" ? There are two use cases:

1. Currently MatchStep "tags" a traverser with traverser.getPath().addLabel(step.getId()). This means that the path data of the traverser is used to store which branches of the match() the traverser has already gone down. This is a hack as now the travesrer.path() has a weird step ID label. Also, its inefficient as the path needs to be searched for the respective "tag." A Set<String> is much more efficient.

2. Intersect/symmetric difference will require some sort of "tagging" as explained in Section 4 of http://arxiv.org/abs/1511.06278. We could, of course, use the MatchStep way of doing things, but again, thats a hack.

3. When we support TINKERPOP3-736, using the "path()-hack" will not work as two branches will have different stepIds but the same tag.

In short, anytime we need to know if a particular branch of a traversal was taken, Traverser.getTag() is our friend.

IMPORTANT: This will go into Traverser.Admin as no user should use these tags. These are simply for internal markup of traverser.