Move from build.scala to build.sbt

This PR moves from a project/build.scala sbt configuration to a build.sbt configuration, such that the project can used as a sub-project in other sbt projects.

This solves #5.

Example of usage in another project:

```Scala
import sbt.Keys._
import sbt._

object MyBuild extends Build {
  lazy val active_slick= Project (
    "active-slick",
    base = file("lib/active-slick")
  )

  lazy val project = Project (
    "awesome",
    file("."),
    settings = Some.Settings.Object aggregate(active_slick) dependsOn(active_slick)
}
```