Decimal: read from Avro

Allows reading decimal columns with or without codegen. Includes tests
based on a data file posted on HIVE-5823.

Change-Id: Ie541c6b98bd24543691850cb45a434af60b5a5a6
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2437
Reviewed-by: Skye Wanderman-Milne <skye@cloudera.com>
Tested-by: jenkins