Relup support for Windows

Hi Tristan,

This PR adds relup support for Windows. I made the interface similar to the shell script one, so it is possible to perform a relup with the following commands:

```sh
$ bin\myrel upgrade 0.1.1
$ bin\myrel downgrade 0.1.0

$ bin\myrel install 0.1.1
$ bin\myrel install 0.1.0
```

`install` without argument installs the service, with an argument performs a relup.

Cheers,
syl20bnr