INT-3027: Headers Enrichment for <enricher>

* Add `<header>` sub-element to `<enricher>`
* Refactoring of `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

INT-3027: add overwrite & type to enricher header

Move `HeaderValueMessageProcessor` hierarchy to 'transformer.support' package

INT-3027: Make header's overwrite=true by default

Doc Polishing