Scala rules override javac args, do not allow extra scalac args

[dsilvasc]

Note how Kotlin and Groovy rules both have extra args for their compilers -- getExtraKotlincArguments and getExtraGroovycArguments:
https://github.com/facebook/buck/blob/master/src/com/facebook/buck/jvm/kotlin/KotlinLibraryDescription.java#L164
https://github.com/facebook/buck/blob/master/src/com/facebook/buck/jvm/groovy/GroovyLibraryDescription.java#L97

The Scala rules don't have that, and they override the javac extra args:
https://github.com/facebook/buck/blob/master/src/com/facebook/buck/jvm/scala/ScalaLibraryDescription.java#L178

This leads to uber/okbuck#645 and uber/okbuck#644