* INT-4376: Upgage to Log4J 2
JIRA: https://jira.spring.io/browse/INT-4376
* Deprecate Log4J 1.x components in favor of newly added
a `Log4j2LevelAdjuster` JUnit `@Rule`
* Update all the logging configuration to Log4J 2
* Polishing after testing
Previously there was a need to add `result-type="StringResult"`
to <int-xml:xslt-transformer> for xslt output `method='text'`
in case payload was non-standard type (e.g. `File`), if we wanted to get
result as text.
* Add coercion to `StringResult` code, if `Transformer`'s property `method` is 'text' and there is no explicit `resultFactory` provided.
* Add documentation to reference manual
JIRA: https://jira.springsource.org/browse/INT-3067
