Make pylearn2.utils.wraps work on abstract methods

[lamblin]

It would be reasonable to have abstract base classes defining methods that have to be implemented by subclasses, and to re-use (part of) the docstring of these methods.
Doing so currently breaks the generation of online documentation, because the base class cannot be instantiated to extract its docstring.
Starting points (indicated by @fvisin): http://code.activestate.com/recipes/576862-docstring-inheritance-decorator/, http://code.activestate.com/recipes/578587-inherit-method-docstrings-without-breaking-decorat/