Updated RC4 with __prev__
Updated RC4 with __prev__

rc4(key) still returns a generator for an RC4 stream, but implemented as a class instance so that the __prev__ method can be included to run the RC4 stream backwards. The __next__ method runs the stream forward. For python2.x, next and prev are synonyms for __next__ and __prev__.
ms0/crypto,ms0/crypto