Migrated to Gradle build

This commit migrates from a Maven-based build system to a Gradle-based
one. Changes include:

- Removed archetype & parent
- Renamed core, support, test, security and xml directories to
  spring-ws-core, spring-ws-test, spring-ws-security, spring-xml
  respectively.
- Moved samples to separate project
  (https://github.com/spring-projects/spring-ws-samples)