added SBT launch scripts for *nix and Windows

I'm including 2x SBT build launch scripts (e.g. [Gradle Wrapper style][1], for self-distributing builds) . They're used exactly like SBT (e.g. ./sbt run).

Strictly speaking, this is a build aid/enhancement, and won't add anything directly to the scala-js-ts-importer conversion process.

Thought I'd offer it as a means of getting into Scala/SBT/scala-js-ts-importer faster (for newbies like me :-P )

Cheers

[1] http://www.gradle.org/docs/current/userguide/gradle_wrapper.html