INT-3069 Add Global Gateway Method Metadata

https://jira.springsource.org/browse/INT-3069

Provide a mechanism to specify headers and payload-expression that can
be applied to all methods in the gateway.

- Headers defined as 'default' are globally applied to all gateway methods.
- Also supports 'default-payload-expression'.
- Headers defined on a specific method override the global settings.
- An `@Header` in the interface is overridden by a specific <header/> for that method (current behavior)
- An `@Header` in the interface is NOT overridden by a  <default-header/>
- Add 3 new SpEL variables:
-- #methodName (synonym for #method - deprecated)
-- #methodString (a string representation of the method showing return type and arg types)
-- #methodObject (the Method object)

	<int:gateway id="sampleGateway"
			service-interface="org.springframework.integration.gateway.GatewayInterfaceTests.Bar"
			default-request-channel="requestChannelBaz">
		<int:default-header name="name" expression="#methodName"/>
		<int:default-header name="string" expression="#methodString"/>
		<int:default-header name="object" expression="#methodObject"/>
		<int:method name="baz">
			<int:header name="name" value="overrideGlobal"/>
		</int:method>
	</int:gateway>

INT-3069 Polishing - PR Comments

- Remove extra 'method' variables, just provide `gatewayMethod`.
- Parser improvements
- Schema now enforces default-header elements to precede method elements
- Doc polishing