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.

INT-2491: Polishing

* Add check for duplication of provided variables
* Tests and Docs

Polishing