Several fixes to get chromium tabs working under OSX 10.8 or later

I think this probably fixes issues #14, #10 and #9.

* Opening a new window and closing it was crashing. The main root-cause
    here was an incorrect autorelease call in the CTBrowser class when
    new windows were created. By allowing the autorelease in windowWillClose
    to be the one that gets rid of the final reference, the bug primarily went away.

* TabStripController referencing deleted memory after CTBrowser being deallocated.
    TabStripController references a C++ class (CTTabStripModel)that is
    non-refcounted and owned by CTBrowser. Once the CTBrowser gets deleted,
    that class reference becomes stale and cannot be safely used again. Since it
    isn't refcounted, the simplest solution is actually just to ensure that
    TabStripController gets released before CTBrowser, and then the bug goes away.

* It's possible for CTBrowserWindowController to trigger a window close inside
    commandDispatch. This in turn can lead to CTBrowserWindowController getting
    destroyed before commandDispatch is finished, with predictably poor results.
    Doing a retain and autorelease on the targetController in commandDispatch
    fixes this.

* Tearing away a tab and then reintegrating it was causing a crash. The basic
    problem was that we were attempting to call makeFirstResponder while in the
    process of closing a window. This was because CTTabWindowController calls
    setUseOverlay in its dealloc routine, which in turn CAN call NSWindow
    makeFirstResponder and display. I've put in an ugly but effective hack to
    prevent this. Ideally I'd check some preexisting state on the window or
    windowcontroller itself to see if it's valid, but I couldn't find anything
    obvious.

Lastly, I also fixed up the XCode project settings to build properly on XCode 4.6
under 10.8. Apple no longer supports building for OSs prior to 10.7, so I updated
the project accordingly.

With this in place I can get the example app working robustly, and should be able
to use this for other purposes as well.