Check for updates through MenuHub's shared item

[mstallone]

• Uses MenuHub 0.3.0: MXSwipe's Sparkle updater is MenuHub's SparkleUpdater, and Check for Updates… moves from MXSwipe's section to the hub. On its own it sits under the version line; in a combined menu one item checks every app, each app shows its own update, and the rest are summed up in one alert.
• MenuHub 0.3.0 is protocol 3, so it combines only with apps on the same version (RetinaShot 1.5.0).
• Sparkle stays at 2.9.4, now through MenuHub.
• README menu images re-rendered.