Add basic Gradle build file #49

New Gradle and old Ant build files exists in parallel. Gradle build
script cannot yet build all artefacts though. Therefore, `build.xml`
should be still used by default.
Temporarily disable "Mutation Coverage" target

In order to simplify transition to the new build system (#49), we simply
remove this auxiliary target. It will be turned back a bit later.
Replace old build tasks with one gradle call
Remove sonar.xml
Detect current version in gradle script
Replace "prepare-release" ant task with gradle one
Unzip swt.jar for windows32