feat: add gradle build script

Add gradle to easily build ag-demo and deploy the resulting war,
also add a configuration example in gradle-conf(example).groovy
and gradlew.sh/.bat for easier execution

addressing the comments by @lucassus in https://github.com/9ci/angle-grinder/pull/113#issuecomment-26442972 I created a new pull request with a cleaner commit history :)