Implement ArangoDB multi-model data store for Apache Gora

Multi model databases are becoming more popular due its native support for polyglot persistence. Polyglot Persistence is a term to mean that when storing data, it is best to use multiple physical data models, chosen based upon the way data is being used by individual applications or components of a single application. Multi model databases acknowledge the need for multiple data models, combining them to reduce operational complexity, operational costs, extensibility and maintain data consistency. Apache Gora currently supports OrientDB datastore as a multi model database, the project proposes further extending multi model database support with ArangoDB