Support for Java 8 optionals

[mouyang]

Now that Optional has been introduced into Java 8, that should be preferred over the opt* methods.

[johnjaylward]

As of now, we are supporting back to java 6

[stleary]

Please see https://github.com/stleary/JSON-java/wiki/FAQ#what-version-of-java-does-this-project-require