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
We typically prepped an underscore to the names of constants that are meant to be used only inside the module and not exported for use elsewhere.