Add support for signed JARs

[Duhemm]

The support for JAR files provided by #942 doesn't include support for signed JARs.

We should port org.apache.harmony.security.utils.JarUtils from Apache Harmony (see here: https://github.com/apache/harmony/blob/java6/classlib/modules/security/src/main/java/common/org/apache/harmony/security/utils/JarUtils.java).

There are already tests for signed JARs in:

• java.util.jar.JarEntrySuite
• java.util.jar.JarFileSuite

How to fix this issue:

1. Fix these issues:

• Port java.security.MessageDigest from Apache Harmony #953
• Port java.security.cert.Certificate from Apache Harmony #954
• Port java.security.cert.CertificateFactory from Apache Harmony #955
• Port org.apache.harmony.security.utils.JarUtils from Apache Harmony #957
• Port java.security.CodeSigner from Apache Harmony #958
• Port java.security.Timestamp from Apache Harmony #959
• Port java.security.cert.X509Certificate from Apache Harmony #960
• Port java.security.cert.X509Extension from Apache Harmony #961
• Port javax.security.auth.x500.X500Principal from Apache Harmony #962

2. Use JarUtils in JarVerifier.verifyCertificate:

https://github.com/apache/harmony/blob/java6/classlib/modules/archive/src/main/java/java/util/jar/JarVerifier.java#L295-L298

        try {
            Certificate[] signerCertChain = JarUtils.verifySignature(
                    new ByteArrayInputStream(sfBytes),
                    new ByteArrayInputStream(sBlockBytes));
            (...)