Push to NuGet on tag, not on commit to master branch

[stakx]

Working on a separate develop branch rather than directly on master has had one big benefit, namely that not every PR merge or other commit immediately triggered a new NuGet package release. We could prepare the next release on develop, and then merge into master to publish. However, maintaining two branches introduced its own problems, e.g.:

• It is hard to predict the version number to be put into CHANGELOG.md, since master has additional merge commits that develop doesn't have, so GitInfo arrives at a different (later) version number on master.

• The master branch is still set up as default, so PRs are often made against the wrong branch.

This PR is a first step towards a different publish procedure: If all goes as planned, we can permanently merge develop into master and abandon it. Commits on master will no longer automatically trigger a NuGet push, only pushing a tag will have that effect. This is especially nice since we're already tagging release commits.