Add the Roman Numeral Kata.

This is my first time through the Roman Numeral Kata.  I was surprised by how simple the solution ended up being; I'd started by tabulating the basic characters (I, V, X, etc.), but then later realized when it came to writing the tests for the subtractive forms that the composite values could be built the same way.  The result is a greedy algorithm equivalent to the coin selection problem.

As a side note, as my first Scala project, I had a bunch of problems getting things set up.  ScalaTest, specifically, never quite worked right in IntelliJ; it wasn't detecting the implicit conversions strings and the `Unit` => `BoxedUnit` conversion for the actual case blocks.  The common concern of tooling in the Scala community still seems to hold.