Relicense examples under CC0

In order to better serve the needs of people looking at the examples, this PR relicenses all of the code under `examples` to the [CC0 1.0 Universal Public Domain Dedication][cc0].

[cc0]: http://creativecommons.org/publicdomain/zero/1.0/

## Current :copyright: holders

*If you would like to manifest your intent to relinquish all copyright claims to your prior contributions to the affected code (everything under `examples`), both code and text, under the terms described in this pull request, please reply with (at least) a :+1: (or a :-1: if you would prefer not to do so).*

- [ ] @ben
- [ ] @arrbee
- [ ] @carlosmn
- [ ] @vmg
- [ ] @kadamski
- [ ] @chris-y
- [ ] @drafnel
- [ ] @kiryl
- [ ] @schacon
- [ ] @schu
- [ ] David Boyce <boyski@users.sourceforge.net>
- [ ] @nulltoken
- [ ] @ethomson
- [ ] @gnprice
- [ ] @sba1
- [ ] @linquize
- [ ] @scottjg
- [ ] @13ren
- [ ] @cscheid
- [ ] @Lionelon
- [ ] @martinwoodward