INT-2491: Scripting: Variables improvements

JIRA: https://jira.springsource.org/browse/INT-2491

* Add support variables for 'inline' scripts
* Add convenient attribute `variables` for script tags
* Introduce `BeanFactoryFallbackBinding` for groovy scripts.

The is no `BeanFactory` for JSR 223, because its engine is very 'aggressive' for variables resolution. First of all it calls `javax.script.SimpleBindings#entrySet()`