code: public Class getClass(String key, Class defaultValue, Class assignable)
      {
          return getClass(key, defaultValue, new Class[]{assignable});
      }
docs: Returns the class specified by the value for the specified key. If no
  value for this key is found in the configuration, no class of this name
  can be found or the specified class is not assignable <code>assignable
  defaultValue</code> is returned.
  
  @param key the key
  @param defaultValue the default Value
  @param assignable a classe and/or interface the specified class must
  extend/implement.
  @return the value for the key, or <code>defaultValue</code>