Master branch renamed to Main

[louthy]

I have renamed the master branch to main. Whilst there may be some debate about whether master in git has any connotation or relation to master/slave, I'd prefer this project stood on the side of history that doesn't think using terms that are potentially loaded is ok.

If someone has a local clone, you can update your locals like this:

$ git checkout master
$ git branch -m master main
$ git fetch
$ git branch --unset-upstream
$ git branch -u origin/main
$ git symbolic-ref refs/remotes/origin/HEAD refs/remotes/origin/main

For open PRs I will see how easy it in to merge everything when I get to them, but I may just ask you to re-push to main.