Automated test of sbt-support, and other fixes

This commit includes the first automated test and other fixes from https://github.com/typesafehub/dbuild/pull/128, plus:
* disable parallel execution in IntegrationTest, since it is currently unsafe to run multiple instances of dbuild in parallel
* fix an issue where additional settings, injected via the "settings" option, are not handled correctly if they refer to build levels not in the actual source code

/cc @jsuereth