Fix YubiSecurityModule.unpad() errors

* Make pad() and unpad() regular methods instead of classmethods,
  because it is more consistent with the rest of the code and is the
  first step towards a more generalized pad/unpad mechanism in the whole
  SecurityModule class hierarchy.
* Remove 'accept_invalid_padding' config parameter because we don't
  want to clutter our config. When incorrect padding is detected a
  ValueError is raised.
* Remove try/except block from unpad() method because there is no
  reasonable way to handle all Exception there. They will be handled
  somewhere else
* Improve pad() and unpad() docstrings

Change-Id: Ic7855ed999df1b406d126761cf54300b983276ee