Fix UITableView observation info getting leaked

ARGenericTableViewController is added as an observer of its
UITableView's `editing` keypath within -viewDidLoad but is never
removed. When the UITableView is deallocated the observer may get
attached to some other object -- this is the source of the following
console warning:

    An instance 0x130a2000 of class UITableView was deallocated while key
    value observers were still registered with it. Observation info was
    leaked, and may even become mistakenly attached to some other object.
    Set a breakpoint on NSKVODeallocateBreak to stop here in the
    debugger. Here's the current observation info:
        <NSKeyValueObservationInfo 0x11a61280> (
            <NSKeyValueObservance 0x11a61220: Observer: 0x11a3d0c0, Key
            path: editing, Options: <New: NO, Old: NO, Prior: NO>
            Context: 0x0, Property: 0x11a612a0>
        )
Merge pull request #1 from andrewsardone/aps-kvo-leak-fix

Fix UITableView observation info getting leaked