make spark 2 work with Java 7 again

move tests to a separate module to be able to run it with a different language level witin IDEA
 - com.sparkjava:tests runs on java 8 to make use of lambdas
 - IDEA does not support different language levels for code and tests, so it has to be in separate modules

changes to spark-core are trivial, just making some variables final as per Java7 requirements