Jar signing and verifying

JAR spec: http://java.sun.com/j2se/1.5.0/docs/guide/jar/index.html
jarsigner:  http://java.sun.com/j2se/1.5.0/docs/tooldocs/solaris/jarsigner.html
Signing on Android: http://nelenkov.blogspot.tw/2013/04/android-code-signing.html