Tinkerpop Blueprints API

Moving the Neo4j implementation to [Tinkerpop Blueprints API](https://github.com/tinkerpop/blueprints/wiki). Now, Activiti runs over Neo4j, OrientDB, Titan and others. Maybe this repo could be renamed to **activiti-graph**.

I strongly believe in the power of graphs and I hope that we can make Activiti know this power. :)