Add profile for signing artifacts

Publishing artifacts to maven central requires signing. To make signing work I
added the following configuration in the $HOME/.m2/settings.xml:

  <profiles>
    <profile>
      <id>gpg</id>
      <properties>
        <gpg.executable>gpg</gpg.executable>
        <gpg.passphrase></gpg.passphrase>
      </properties>
    </profile>
  </profiles>
  <activeProfiles>
    <activeProfile>gpg</activeProfile>
  </activeProfiles>

as documented in [1]. Note that the executable could be named gpg or gpg2
depending on the platform.

Use the following command to build, sign and stage the artifacts to the OSS
Sonatype:

mvn clean deploy -DperformRelease=true \
  -DaltDeploymentRepository=sonatype-nexus-staging::default::https://oss.sonatype.org/service/local/staging/deploy/maven2/

[1] https://gerrit-review.googlesource.com/Documentation/dev-release-deploy-config.html

Change-Id: I4faede2f754656772320c326f4890e2c74a9dce0