HV-920 Adding a regular expression based URL constraint validator which ...

...needs to be configured via XML or via a ConstraintDefinitionContributor
Btw. it can alternatively be written very nicely using new lambda style:

    	HibernateValidatorConfiguration configuration = Validation
		.byProvider( HibernateValidator.class )
		.configure()
		.addConstraintDefinitionContributor( builder -> {
			builder.constraint( URL.class )
				.includeExistingValidators( false )
				.validatedBy( RegexpURLValidator.class );
		} );

Nice how pieces fall into the right places :)