[feature] set up allowed class-permissions to JEXL constructor

[Milchreis]

hi,

I had some trouble by using commons-scxml, because JEXL doesn't know my objects. Unfortunately JEXL is not very verbose and I took some time to understand that there are permissions. commons-scxml has no "easy" way to add some custom permissions and it needs a deeper understanding to see that the JexlEvaluator has to derive with an overridden createJexlEngine-method.

This PR contains a new constructor for JexlEvaluator which allows a custom set of classes which is used in createJexlEngine(). Moreover there is a test now, which supportes the usage of classes outside the allowed java- and sxcml2-packages.

I hope it finds a way into this great library 😊