Enable SNAPSHOT uploading through ant target.

This changeset adds an additional ant target which deploys snapshot
releases directly into the sonatype snapshot repository.

See https://docs.sonatype.org/display/Repository/Sonatype+OSS+Maven+Repository+Usage+Guide#SonatypeOSSMavenRepositoryUsageGuide-7c.DeploySnapshotsandStageReleaseswithAnt
for more details on how to setup local authentication.

Change-Id: I3dc26b70c3547e42c0181486bc4e189b5e30ace9
Reviewed-on: http://review.couchbase.org/33881
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>