Improve model converters, build a Scala Model Converter

[fehguy]

It's pretty much impossible to support scala without referring to scala types directly, which means bringing in scala libraries. To keep things lean, it would be best to have a single ScalaModelConverter class, which not only loads the DefaultScalaModule from Jackson, but handles scala types as needed.

• converter/ScalaEnumTest.scala

[fehguy]

I've moved scala support into a separate module so it can both evolve independently and not clutter up the java code.

See https://github.com/swagger-api/swagger-scala-module

When this module is included as a dependency, it will automatically be detected and enabled by the ModelConverters.