Turned TitanGraphSerialTest into a Groovy class
Serial performance test refactoring

* Added configurable number of edge labels to GraphGenerator

* Added primary keys to GraphGenerator's edge labels (only simple keys
  are implemented right now -- no composites)

* Added gremlin-groovy dependency to titan-test and added a
  Gremlin.load() static call in GroovySerialTest

* Added a test on vertex-centric indexing using a primary-keyed edge
  label in GroovySerialTest

* Started refactoring GroovySerialTest to look less like Java and more
  like Groovy, but that needs additional work