CC-BY-3.0: add the full text of the CC-BY 3.0 unported

CC-BY-3.0: add the full text of the CC-BY 3.0 unported

The license content is owned by Creative Commons and is distributed
under the CC BY 3.0 unported.

From http://creativecommons.org/terms:
> 8. Terms Relating to Content on the Websites and Services.
>    b. Licenses Associated With Content on the Websites and Services.
>       i. CC Content: All Content (other than computer software) owned
>          by Creative Commons and made available by CC on the Websites
>          or through the Services is licensed under the Creative
>          Commons Attribution 3.0 Unported license, unless marked
>          otherwise.

I've converted the HTML version from
http://creativecommons.org/licenses/by/3.0/legalcode to text, but
haven't made any other changes.

Signed-off-by: W. Trevor King <wking@tremily.us>