Added windows service package

* moved upstart to service/upstart
* added windows service management package.

PR notes
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This PR adds a service package and moves the upstart package under it. The Conf type has been moved to service/common as it will be used by the windows service package as well, and all methods attached to it have been migrated to the Service type. This way we avoid depending on a separate type for installing a service, and relying on the service type for removing itself.  

Windows service package notes
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The current implementation installs the service as the user under which juju is currently running. This is done because some installers in windows expect that a real human will run it. The "system" user under which services usually run, does not set a proper user profile as well as other attributes, and that might cause some installers to fail.

The downside of this is that to install a service under a password protected user, you need the password for that user. The password is saved as a secure string decodable only by the juju user. The windows userdata script does save the password inside C:\Juju\Jujud.pass. The Install() method reads the encrypted password in that file and decodes it. It is the used to create the service.