Rename pipeline to traversal

https://www.pivotaltracker.com/story/show/82154024

Please remove this, as the URL is not valid. FYI I believe in the new architecture the best reference URL is simply:
https://github.com/tinkerpop/tinkerpop3/blob/master/gremlin-core/src/main/java/com/tinkerpop/gremlin/process/Traversal.java