Fix issue 339 spock-maven plugin.xml now based on version configured in maven.gradle

- Applied property filtering for all versions in plugin.xml so they are maintained in maven.gradle
- Extended README.md to indicate spock-maven only works for Maven 2.x
- Added spock-maven/README.md to explain how to use the plugin with Maven 2, how to use Spock and Maven in general, and how to configure surefire to include *Spec.groovy files