first pr of Groovy-Optional

Last year I proposed a way of transparent handling of java.util.Optional.

http://groovy.329449.n5.nabble.com/Groovier-way-of-handling-Java8-s-java-util-Optional-td5717544.html#a5717561

I think this is very groovy way to handle Optional value, although Optional itself might not be good idea compared to safe navigation, anyway.
I imagine you groovy development team are tackling Java8 aware GDK methods?

So I rewrite above proof of concept code to actual PR to current Groovy
implemented as a GDK method java.util.Optional.methodMissing().

I hope reviewing this, and please advice me about anything to do.
(for example where is the right branch to rebase?)
Probably it is needed to add custom type checking rule to avoid STC errors
which comes from use of Optional.missingMethod.

Best regards,