add appveyor script

If the assembly version changes you need to ensure that the baufile is run so that the version gets updated correctly in the appveyor.yml file.
I don't think we should ever be setting the fourth version part. This is a .NET/NuGet oddity and not part of SemVer.

For dev builds being pushed to myget, we should probably use something like `0.5.0-{some pre release identifier}-%APPVEYOR_BUILD_NUMBER%`. The problem then is that we don't want that to happen for the master branch builds, otherwise we can't push the artifacts to nuget.org. Also, how do we build beta releases from dev to push to nuget.org?

The strategy I'm using in other projects is to always build without a version suffix by default, but when I want to push a beta to NuGet, I set the VERSION_SUFFIX environment variable via the GUI and then grab the artifacts and push. Ideally, this would be just another config in the build pipeline in TeamCity that grabs the artifacts from the CI build, alters them and pushes them. However, this is beyond the limits of simple 'one build config per repo' solutions like Appveyor and Travis CI. I think they are both fine for simple scenarios but break down quickly as soon as you need anything like a build pipeline. Anyway, this is all overkill for a simple project like ColoredConsole. I'm wondering if the burden of pushing to myget is actually overcomplicating things. Perhaps we should drop it? At the moment I only envisage small revs and almost immediate releases, so does myget really add any value here?