Replacing inheritance with decorators
Replacing inheritance with decorators in luigi.util

- Added docstring to @inherits (thanks, alexbw) and deprecated the Derived class
- Added a @requires decorator
- Added some helper method for tasks using @inherits
- Added a @copies decorator and deprecated the Copy class
- Added a @delegates decorator and deprecated the CompositionTask class
Merge pull request #218 from erikbern/decorator-fixup

Refactored luigi.util and deprecated Copy, Derived etc