Add BSD-2-CLAUSE license to head of each file

I've used a ruby gem called 'copyright-header' available from
https://github.com/osterman/copyright-header.

There was a bug do do with their --remove-path flag, I've submitted a
pull-request for it, but if you do need to use this script with that
flag before it's landed, you can install my version with:

```
git clone git://github.com/caitp/copyright-header.git -b remove-path

cd copyright-header
gem build copyright-header.gemspec
sudo gem install copyright-header-<version>.gem
```

Hopefully they accept my patch some day, but there doesn't seem to
have been activity for a few months.

Corrected copyright year, removed README.md from

Removed README.md from relicense.sh default actions

README.md must now have its license updated manually, as it is to
appear at the tail of the file rather than the head.