Decimal implementation.

This patch implements decimal support for text based formats.

Change-Id: I8e2c9e512ed149fe965216a72cb21fffd4f18e75
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1669
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: Nong Li <nong@cloudera.com>