Use managed source directories to be inline with other sbt code generators.

Here is how sbt-avro does it, https://github.com/cavorite/sbt-avro/blob/master/src/main/scala/sbtavro/SbtAvro.scala#L67

It interacts better with the sbt-idea plugin when you structure it like this too.  I would also recommend changing `/ "java"` javaSource to something more like https://github.com/cavorite/sbt-avro/blob/master/src/main/scala/sbtavro/SbtAvro.scala#L57

Thoughts?