Changes the build process

* Uses [PSake](https://github.com/psake/psake) to build the project instead of old-fashionned batch file
* Runs the unit tests as part of the build process
* Adds a build task that runs code coverage (it takes forever to run so I split it from the regular tests)

It would even be possible to automatically generate the nuget packages in AppVeyor, using the AV build number and calling the psake task with the `-properties` parameter. AV publishes a NuGet feed, so it's even possible to use it as beta channel.