Fix command substitution bug in checksocket.sh
Additional addVertexWithLabel doc and refactoring

* Created addVertexWithLabel(VertexLabel) in TitanGraph for symmetry
  with TitanTransaction

* Updated labeled addVertex calls in the docs

* Updated the section in docs/titanbasics.txt that specifically covers
  vertex labels and their creation

* Updated javadoc for addVertexWithLabel methods in both TitanGraph
  and TitanTransaction