Add script to automatically update package with the latest version available

I've created a small script that checks `http://mirrors.jenkins-ci.org/windows/latest` for the latest available version and updates the nuspec and installer files if it finds one. It does this without downloading the latest zip archive and it runs pretty quick.

It would be useful to run this script periodically, perhaps from a Jenkins job. I'd be happy to set this up for you, just let me know.

The script requires that the commands `git` and `curl` are available; which might not be the case on every system.

I've placed the update script `update-jenkins.ps1` in the root of the repository; I don't know if that has any consequences for your chocolatey package source.

*warning*
I think this is useful, but I also wrote this to get myself more familiarized with PowerShell, so there might be some issues with my script (although I think it's pretty clean; perhaps a bit too verbose) and a review would be appreciated.