Add BlobStoreBench to jclouds.

BlobStoreBench is simple utility to benchmark blobstore performance.
It uses the basic jclouds constructs to perform GET, PUT, DELETE
operations in a blobstore.

See the convenience script blobstorebench.sh to see how to run the
tool.

Tested READ/WRITE against atmos, aws-s3, azure, cloudfiles-us, s3
(api), hpcloud, filesystem, transient, swift-keystone.