INT-3139: Add #jsonPath SpEL Function Support

* #jsonPath function is registered automatically if the library is on the classpath
* 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`

INT-3139: Polishing