Extend build.xml to support maven central publishing.

Change-Id: I40343dbbf78b370b09c008a95ab77cd9cf8bfeab
Reviewed-on: http://review.couchbase.org/26714
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Reviewed-by: Deepti Dawar <deepti.dawar@globallogic.com>