Added GreetingsSource Java interface and basic Groovy implementation,
with tests
Refactored sayHello method, which now calls sayHelloTo
Refactored tests providing common field for GreetingsSource under test
BasicGreetingsSource needs now greetings initialization
Changed class (and test class, too) name, in order to address new
implementation feature
Using Groovy compiler for both src/main/groovy and src/main/java, in order to allow joint compilation. Reformatted Gradle build file in Groovy style
Crazy Java implementation for GreetingSource: Java extends Groovy implements Java