Remove unused loggers variables
Improved code readability

  * Added basic comments to organize the file.
  * Added `defaults` module to facilitate option management.
  * Fixed an issue where the fast path creation would never be enabled
    if any kind of option was passed to `createLogger`.
  * Moved `module.exports` to the end of the file.
  * Updated error message of `init` to be more concise.
Update mocha@2.0.1
Disable logging by default

Starting on `bunyan@1.2.2`, it is possible to (again) disable logging by
defining a level higher than `bunyan.FATAL`. This is now the default as
it more accurately represents what the inexistence of the `DEBUG` env
var has on logging when using the `debug` module - no output is
generated by loggers.