Move public API classes into com.cloudera.cdk.data.dao package. This is
not necessarily the final name, it was chosen just to group the API classes
together and may change later.
Group HBase javadoc
Reduce package visibility and move non-public Avro classes into an impl
package so they are hidden.

The only public packages are com.cloudera.cdk.data.hbase.{avro,manager,tool}.