Automatically instantiate GremlinScript when calling .exec() or .fetch()

[jbmusso]

gRex currently requires passing an instance of GremlinScript to client.exec() or client.fetch(). These two methods should be able to instantiate a new script themselves.

This will make the API smaller for one line scripts, which are typical for simple queries/traversals.

The following:

client.exec(gremlin(g.V())).done(...);

may then be replaced by:

client.exec(g.V()).done(function(res) {