HV-828 Introducing ConstraintValidatorLocator to allow for a pluggable d...

...iscovery of additional constraint validator implementations.
The general idea of the locator is great, but I'm wondering whether we can reduce verbosity of implementations a little bit.

So I've played with the idea of providing a fluent API for configuring constraint definitions. You can find the commit [here](https://github.com/gunnarmorling/hibernate-validator/commit/b017e9f9209ab002cdafcfe394a84d8adaab365a). This reduces this implementation to the following lines, which I think are nicer to write and read:

	public void getConstraintDefinitions(ConstraintDefinitionBuilder builder) {
		builder
			.constraint( AcmeConstraint.class )
				.includeExistingValidators( keepDefaults )
				.validatedBy( AcmeConstraintValidator.class )
			.constraint( AcmeConstraintWithDefaultValidator.class )
				.includeExistingValidators( keepDefaults )
				.validatedBy( DefaultAcmeConstraintValidator.class );
	}

WDYT?

Btw. I used the terms ("constraint definition", "includeExistingValidators" etc.) known from the XML configuration, I think it'd be nice to stay consistent.