Modularize build, migrate to sbt 0.13.1, enable Travis CI

This makes me wonder, is there a way we can run sbt in "minimal mode", where it only loads build.sbt, but no plugins / other *.sbt files? This would probably be useful for dbuild, if we stick to the convention that the core build defines what's necessary for building/testing/publishing, and the other .sbt files are optional.

review by @jsuereth, @vjovanov