Adding "debug" module as a dev-dependency

The "debug" module allows debug information to be output into the
console when process.env.DEBUG="Sequential:<selector>" where
<selector> is one of the following:
  - "*" (shows ALL debug information)
  - "Generator" (shows debug information only from the Generator)
  - "Accessor" (shows debug information only from the Accessor)

Note that the module is a dev-dependency and not a dependency. If
the "DEBUG" environment variable is set without the module being
installed, an error will occur