External Data Source: Few small API changes

* Rename getStats() to prepare()
* Adds TRowBatch.num_rows to indicate number of rows when no cols are
  materialized
* Changes api and sample poms to produce source jars

Change-Id: I02dcc89e27716978708386cfc3f7940ee5dbc023
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2406
Reviewed-by: Matthew Jacobs <mj@cloudera.com>
Tested-by: jenkins