Update push example - tiny
Add javadoc overview and package desc (simple yet)
Update README.md
Update README.md
Update pom parent
[maven-release-plugin] prepare release v3.1.1
Update maven SNAPSHOT for maven release
[maven-release-plugin] prepare release v3.1.1
Fix bug:  registrationId method - typed with ALIAS
Fix bug - null extras will cause IllgalArgumentException, android notification.anroid() will enter into this situation.