Add SBT without changing directory structure.

So maybe this works out:
I've configured SBT so that it sticks with the existing directory structure.
Compiling, running tests and packaging works and it doesn't interfere with the existing ant build.
So if it's OK for you to have these 3 additional files hanging around then people can choose between ant and SBT.

I could also create a task that downloads dependency jars specified in `build.sbt` files into the `lib` directory so that you don't have to download them manually when switching versions / adding new dependencies.

What do you think?