added async support for exec:java

[koh-osug]

I had the problem of a server started with exec:java blocking the further execution of Maven after the pre-integration phase. The current plugin seems to offer no suitable way of executing a Java main class and let the execution continue while Maven can continue.

This patch supports an async option together with a flexible asyncStartCheck option to be able to provide a custom script evaluating if the started Java class is ready. It is using the JavaScriptEngine for this evaluation supporting a wide range of script languages. I added a test using Groovy as project16. I had to update the Groovy version of some plugins, the used version was very old.