Fix #684: Clean up sbt plugin

- Move things we don't want to be imported into *.sbt files
  automatically to a separate object
- Classify keys using KeyRanks
Merge pull request #767 from gzm0/cleanup-sbt

Fix #684: Clean up sbt plugin
Implement JLong.numberOfTrailingZeros
Merge pull request #769 from alexander-myltsev/trailing-zeros

Implement JLong.numberOfTrailingZeros
Cleanup Scala.js build
Merge pull request #770 from gzm0/clean-build

Cleanup Scala.js build