INT-3297: Add `@MessagingGateway` support

JIRA: https://jira.springsource.org/browse/INT-3297

* Add `<gateway>` annotation analogue and its `MessagingGatewayRegistrar` to register `GatewayProxyFactoryBean` based on annotation attributes
* Add `@IntegrationComponentScan` and its `IntegrationComponentsRegistrar` to scan packages for integration components
* Add implementation to scan `@MessagingGateway`
* Add simple `@MessagingGateway` test