Installer signing

[embray]

The official build of DockerToolbox.exe is properly signed to originate from Docker Inc. Obviously we will want to do the same. The application signing appears to be done separately from the actual build of the .exe. Will have to find out how to do this.

Update: Another note on signing: Powershell scripts can and should be signed as well. Currently we do not sign the scripts for launching Docker/Sage, but we could just as well (in the meantime this is easy to workaround by running powershell with -ExecutionPolicy Unrestricted in the launcher shortcuts)