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.
It's not hugely important, but scriptcs installation should really go under `:install` - I think this will allow Appveyor to convey more semantics when it goes wrong, i.e. 'installation step failed' or something.

Oh, and the casing is `scriptcs` :wink: