More explicit and verbose scalac and javac options

Simple commit to enable few more warnings:

  - all recommended for any Java compiled files
  - 3 extra warnings for Scala (they helped me to eliminate some non optimal code by pointing potential mistakes, they turned out to be quite useful)

A be little more explicit about source and target assumptions:
  - encoding for Scala files
  - explicit target for Scala output (JVM 1.6)