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

**REVIEW ONLY**
There is need:
* to improve `IntegrationComponentsRegistrar`, to make it more generic and independent of cocreate `Registrar` implementation - Visitor pattern
* Conver `GatewayParser` to use `MessagingGatewayRegistrar`. It's allow to check existing tests that everything is correct in the implementation of latest
* Add more test
* Add docs