INT-2433: Get Rid of MessageProcessors wrapping

Previously, provided `MessageProcessor` to create a `MessageHandler`
for endpoint was wrapped to `MethodInvokingMessageProcessor`,
e.g. `GroovyScriptExecutingMessageProcessor`.

Check the type of provided object in the constructor
of `MethodInvoking*` strategies and use it directly if it is `MessageProcessor`

JIRA: https://jira.springsource.org/browse/INT-2433
INT-3139: add #jsonPath SpEL function support

* Add `<json-path-spel-function/>` component
* Add otional dependency on `com.jayway.jsonpath:json-path` artifact
* Add `JsonPathUtils` class as a wrapper around `JsonPath`
* Add `#jsonPath` tests for: `<transformer>`, `<filter>`, `<splitter>`, `<router>`

JIRA: https://jira.springsource.org/browse/INT-3139
INT-3139: Further improvements

* Remove `SpelFunctionRegistrar`
* Introduce `SpelFunctionFactoryBean`