Rewrite of the main README.

Have done a rewrite. Removed what was essentially my original specification with more of a functional/conceptual description. Am also working on a simple worked example that shows off the CLI (stay tuned for another PR).
> I think this change has been merged to rename this

Just yesterday.

    ivory snapshot --repo ./hippay --tombstone 'NULL' --delim ',' --date 2014-06-01 --output dense:psv=snapshot_20140601