moves everything to an sbt style

## motivation
Projects are more accessible if they use standard tooling and organization.  For better or for worse, sbt is the standard in building projects in scala, and so it's much easier for people to use probability-monad if it interoperates with sbt.

## implementation
added a build.sbt file, chose a scalaVersion, moved everything to a namespace, and moved the boot.scala code into an initialCommands setting in sbt.

## feedback
I just guessed on most of these default settings.  If you don't like any or all of them, let's chat and work out something amenable.  I think this project is awesome, and I'd be happy to contribute to other things too.  This just seemed like an obvious place which might be useful.  If you decide you absolutely don't want sbt, we can chat about that too.