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
INT-2867: Add Script Support to inbound-c-a

* Add `<script>` and `<expression>` to `<int:inbound-channel-adapter>`
* Refactoring for `DefaultInboundChannelAdapterParser`
* Introduce `ScriptExecutingMessageSource` to allow scripts' `MessageProcessor`
implementations work as a source for polling endpoint

JIRA: https://jira.springsource.org/browse/INT-2867

INT-2867: documentation

In addition the tests for Lifecycle attributes

Polishing
DOS->Unix NewLines in Several Core Files
INT-3027: Headers Enrichment for <enricher>

* Add `<header>` sub-element to `<enricher>`
* Refactoring to `EnricherParser`
* Add Headers Enrichment logic to `ContentEnricher`
* Add tests
* What's new and Content Enricher's `<header>` note

JIRA: https://jira.springsource.org/browse/INT-3027
INT-3027: Addressing PR comments
INT-3027: add NPE asserts to map properties