Update the Updater

This PR fixes #85, allowing users to update Update.exe themselves (as well as solving the Which Came First problem of trying to update Update.exe while executing the old Update.exe).

To use this, include "Squirrel.exe" in a package - your users' local copy of Update.exe should be revved to the new version. Ideally this should be a pretty rare scenario post-1.0, as Update itself is ideally pretty boring / sparingly used.