Add reference docs tasks to Gradle build

- Move spring-webflow-reference to src/reference
- Upgrade to Docbook 5
- Apply docbook plugin
- Add tasks for generating distribution zip files

spring-binding/build.xml

@@ -1,9 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<project name="org.springframework.binding">
-
-	<property file="${basedir}/../build.properties"/>
-	<import file="${basedir}/../build-spring-webflow/package-bundle.xml"/>
-	<import file="${basedir}/../build-spring-webflow/publish-artifact.xml"/>
-	<import file="${basedir}/../spring-build/standard/default.xml"/>
-
-</project>