Beta Plugin: Implement register_uninstall_hook for cleanup

Something like:

```
function jetpack_beta_activate(){
register_uninstall_hook( __FILE__, array( 'Jetpack_Beta', 'unistall' );
}
register_activation_hook( __FILE__, 'jetpack_beta_activate' );

// And here goes the uninstallation function:
function uninstall(){
// codes to perform during unistallation
delete_option( self::$option_dev_installed );
delete_option( self::$option );
delete_option( 'jp_beta_autoupdate' );
}
```

As requested in https://github.com/Automattic/jetpack-beta/pull/44#issuecomment-356071265

No file or test is named. Start by locating the Jetpack Beta activation entry point and the existing uninstall-related code, then compare it with the requested register_uninstall_hook example. Done means the uninstall callback is registered and removes the three listed options; verify the behavior with the plugin's available tests or manual uninstall flow.

Written by the indexing model from the issue text.