More (negative) tests

[axman6]

I looks like there's a lot of good documents being used for testing, but there can always be more. In particular, it would be good to add tests from the following few documents:

http://www.cl.cam.ac.uk/~mgk25/ucs/examples/UTF-8-test.txt - has many invalid byte sequences, and explanations for why they're invalid.
http://www.columbia.edu/~fdc/utf8/
https://www.w3.org/2001/06/utf-8-test/UTF-8-demo.html
https://github.com/bits/UTF-8-Unicode-Test-Documents - has documents with all valid unicode code points

Mainly adding this so I have the links saved somewhere.