Documentation for PyYAML and libyaml

Update for a PyYAML release

Copy all release artifacts to download/pyyaml
Update wiki/PyYAML.md "Download and Installation" and "History" section
Update wiki/index.md News section

Run

make update

Check diffs, commit and push.

Update for a libyaml release

Put the resulting tarball & zip in download/libyaml
Update wiki/LibYAML.md with release number and date
Update wiki/index.md New section

Run

make update

Check diffs, commit and push.