automatically register codec to make shorthand methods easier to import

I was thrown off when I tried copying-and-pasting the shorthand example:

>>> from rotunicode import ruencode
>>> ruencode('Hello World!')

only to discover, I still had to manuall register the codec -- this attempts to fix it.

Alternatively, the ruencode/decode methods could bypass the codec register and call the underlying methods directy without causing the codec registry to be indirectly modified.

Cheers,
Alex