Accumulo Storage Backend

This fork contains a new Accumulo v1.4.3 storage backend for Titan.

The Accumulo backend is organized as two modules: core and iterators. Titan-accumulo-core contains classes implementing the Titan storage API. Titan-accumulo-iterators provides a column filter used for slice queries, and the Accumulo storage manager may be configured to run this iterator on client or server side.

This initial version does not make use of Accumulo's cell visibility mechanism.