INT-3274: build.gradle: BUILD-SNAPSHOT condition

JIRA: https://jira.springsource.org/browse/INT-3274

* Change the `libs-snapshot` repository to `libs-milestone`
* Add condition on `project.version` to check `BUILD-SNAPSHOT` release category
and enable `libs-snapshot` repository
* Remove redundant directories from `mqtt` module
* and one more project property for Spring AMQP version

To test it om master:
1. `gradle.properties -> version=4.0.0.M3`
2. `gradlew clean test --refresh-dependencies -PspringAmqpVersion=1.2.1.RELEASE`
INT-3130: Add result-type to <o-to-j-transformer>

JIRA: https://jira.springsource.org/browse/INT-3130

* add `result-type` attribute to the `<object-to-json-transformer>`
based on enumeration values: `STRING`, `NODE`
* Provide implementations for methods to convert value to the node representation
* Refactoring according new generic option in the `JsonObjectMapper`
* Add tests and documentation

Need Migration Guide note according this refactoring
INT-3130: remove some deprecation and JavaDocs

Addressed PR comments