Updated license to include CC BY 4.0 for documentation

Updated license to include CC BY 4.0 for documentation, examples, and other materials. This PR simply adds the text for CC BY from:

http://creativecommons.org/licenses/by/4.0/

as plain text, from:

http://creativecommons.org/licenses/by/4.0/legalcode.txt

It is a bit long (as compared to the BSD), but probably should appear complete.
-> IPython source code and examples are licensed ....