Add Accumulo as a backing data store

This series of patches adds support for Accumulo to YCSB. It's based on the Accumulo client from YCSB++.

This has been tested to work with both Accumulo 1.4.5 (commit a3cf980) and the forthcoming Accumulo 1.6.0.

Note that Accumulo 1.6.0 is currently going through release candidate voting. This pull request shouldn't be merged until that release is finalized.