Ready: Update comparison docs

- [x] Neo4j
- [x] MongoDB
- [x] HBase
- [x] DynamoDB
- [x] CouchDB
- [x] Couchbase
- [x] Cassandra

This PR seeks to accomplish two things:
1. To upgrade the Riak descriptions to 2.x
2. General clean up

It does *not* update the descriptions of the other databases. That will be accomplished in a future project.