Histograph Neo4j plugin v0.5.0

Histograph Neo4j plugin v0.5.0:

• /histograph/ping: responds with pong if plugin is properly installed
• /histograph/expand: runs a breadth-first search over hg:sameHgConcept, and returns all PITs in a single concept

v0.5.0 of the plugin expects Neo4j version 2.2.x.