Major refactor

• Change to proper callback structure (callback as the second argument to dependents, not as a success property of the first argument)
• Main export is now parallelized
  • Previously, only the binary was parallelized using clustering
  • Now, the whole library is parallelized (if the number of files meets/exceeds the threshold) using forks not clustering.
• Removal of the dependency lookup shell script in favor of the binary in node-module-lookup-amd
  • Didn't belong there to begin with but was convenient.
• Major refactoring for testability
  • Test coverage fell because it wasn't originally including the bulk of the parallelization logic in the binary.