Remove sbt-assembly

A Scala library with no main class has no
buisness being bundled up into a JAR
containing the Scala Standard Library.
Use newer-style build.sbt plugin settings import
Use sbt settings (not pomExtras) where posssible
Don't crash sbt when not on a branch
Use sbt-pgp v0.8 to prevent publish-local errors
Merge pull request #52 from clhodapp/cleanup/build-cleanup

Build cleanup