Override ConfigObject toString() and add new prettyPrint() method

Override ConfigObject toString() to print it as map to have consistency behavior with println(config) method call
and add new prettyPrint() method
Merge pull request #539 from armsargis/GROOVY-7183

GROOVY-7183: Override ConfigObject toString() and add new prettyPrint() method
Update to the style guide
GROOVY-7184: Type checker incorrectly allows assignment to interface because of RHS implements map