Add scala assignments (Don't merge yet)

The file structure for this is slightly different. The command to run the tests is `sbt test`, and the file structure is as follows:

- `build.sbt` (this is the same file for all exercises)
- `src/test/scala/FooSpecs.scala`
- `src/main/scala/Foo.scala`

I've placed `example.scala` in the root, as with other languages. The file names aren't important, so I can have the test file name match the slug if that makes things easier.