Add support for old-style exception objects. (#58)

Python 2 permits raising old-style objects (objects that aren't
instances of the object class).  Because these objects are not
instances of BaseException, clauses like 'except BaseException:'
don't catch them.

Add support for handling old-style exception objects by:

  * using a class-less 'except:' clause to catch old-style objects
    raised by tasks and done callbacks
  * using the proper exception object class type when raising a stored
    old-type exception during result()