Make the source layout satisfy standarts

We want to replace old Ant build tool with one of modern ones (#49). In
order to make this move more smooth and painless, we must change source
and build files layout, making them closer to the contemporary
standards.
Visualize current target dependencies
Add basic Gradle build file
Define dependencies for Gradle
Define new targets
Use gradle to clean resources
Use gradle to build java classes
Use gradle to build java test classes
Disable "Mutation Coverage" target
Update task dependency tree
Replace old build tasks with one gradle call
Update task dependency tree
Replace old build task with one gradle call
Remove unused definitions from build.xml
Update task dependency tree
Remove sonar.xml
Detect and print version in gradle script
Detect current version in gradle script