MSHARED-306 - keystore and alias parameters can be used for both sign and verify jarsigner tool
+ add a little file in the jar to sign (jarsigner 1.7 consider it as not signer if jar is empty)

git-svn-id: https://svn.apache.org/repos/asf/maven/shared/trunk@1541710 13f79535-47bb-0310-9956-ffa450edef68