SLING-2803 - Create a Validation module capable of validating POST requests or Resources

* API improvements / changes:
        - ValidationModels can describe resource trees
	- removed the ValidatorArgument object which was passed in lists in favour of a simple key-value arguments map
	- created a ValidatorException that can be thrown by Validators in case the validate method is called with missing but expected arguments
* CORE (default implementation):
	- implemented API changes
	- implemented validation models cache invalidation in ValidationServiceImpl
	- implemented validation models overlaying in ValidationServiceImpl (/apps/sling/validation/models vs. /libs/sling/validation/models)
	- provided a default RegexValidator that is more flexible than the previous AlphaCharactersValidator
* TEST SERVICES / IT
	- separated the test services from the integration tests by creating two additional submodules:
		1. test-services - some test services needed for triggering the validation code
		2. it-http - a jar containing integration tests that are run automatically on a customised Sling Launchpad