INT-1639: Add SpEL-Function support

* Introduce `<spel-function>`
* Introduce `integrationEvaluationContextAware` infrastructure
* Some refactoring according new infrastructure
* Some tests

JIRA: https://jira.springsource.org/browse/INT-1639, https://jira.springsource.org/browse/INT-2602
INT-1639: Add <spel-function> support

* add `<spel-function>` XSD element
* add `SpelFunctionParser`
* add `SpelFunctionRegistrar` to avoid introducing some confused 'Method'-bean
* add `SpelFunctionRegistrar` collaboration with `IntegrationEvaluationContextFactoryBean`
* some refactoring for `IntegrationEvaluationContextFactoryBean`
* polishing some failed tests after this change

JIRA: https://jira.springsource.org/browse/INT-1639