Padding in function of the maximum level length at the Transport level.

This is an attempt to address #352.

* `Transport.levelPadLength` is set directly on the `Transport` instance, if you would rather I make a setter (`Transport.prototype.setLevelPadLength`) I have no problem with that, but I didn't see any setters so I didn't create one.
* I updated the provided `Transport`s to use this, and it is somewhat backwards compatible with third-party `Transport`s in the sense that they won't break or crash, but will act as if `Logger.options.padLevels` was ignored.
* It seems that the tests do not strictly validate the raw output of the log calls, but are rather provided a mean to validate the message content. Hence, I did not write any test.

Let me know if there is anything to modify or if you want to discuss the approach!

Thank you