Support OS X Notification Center.

For OS X Mountain Lion, notifications could be optionally delivered
through OS X Notification Center instead of Growl by setting a boolean
variable 'show_notifications_via_notification_center' in 'squirrel.yaml'
which is detailed below.

Added a boolean variable 'show_notifications_via_notification_center'
in 'squirrel.yaml'.
When this variable is set to 'true', 'show _notifications_when' is not
set to 'never' and the OS version is at least 10.8,
notifications will be delivered throug OS X Notification Center instead
of Growl.