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
INT-3297: Further improvement

* Make `GatewayParser` to delegate the hard work to the `MessagingGatewayRegistrar`
* Add Docs
* Checked all existing tests