Add overlooked Foundation files from maccore to the build.

It appears that the following files from maccore were not included in the build. NSUbiquitousKeyValueStore in particular is needed in order to gain access to setters for iCloud key/value storage.

NSAttributedString
NSLocale
NSMutableSet
NSUbiquitousKeyValueStore
NSUuid

The NSDistributedNotificationCenter.cs file was also not included, but it does not appear to contain any needed methods and so was not added back.
Annotate a few AppKit members with ThreadSafe

See comments on this commit:
https://github.com/mono/monomac/commit/2a986b2286adae0695cb26722483ca9b57e0172e

And bug 10604:
https://bugzilla.xamarin.com/show_bug.cgi?id=10604
Add new Mountain Lion properties to NSColor
AppKit: add NSPopover notifications

Consolidate Notification EventArgs augmenting classes into
EventArgs.cs (similar in spirit to Enums.cs) since this is
going to be a common pattern.
AppKit: more notifications

- NSScroller:
  - NSPreferredScrollerStyleDidChangeNotification
- NSView:
  - NSViewFrameDidChangeNotification
  - NSViewFocusDidChangeNotification
  - NSViewBoundsDidChangeNotification
  - NSViewGlobalFrameDidChangeNotification
  - NSViewDidUpdateTrackingAreasNotification
MonoMacGameView: use NSView.GlobalFrameChangedNotification
AppKit: make a number of types partial
NSImage: add CGImage property to match iOS
NSTableColumn: add explicit NSString constructor

Fixes an API break when the System.String constructor
was added where there was an ambiguity between the
NSObject constructor and the implicit conversion from
NSString to System.String.
Include full namespace in error message about Class.ThrowOnInitFailure.

Hopefully avoids confusion like the comments here: http://stackoverflow.com/a/15524950/183422
Remove NSMenuView initAsTearOff as it's not supported on OSX (and it's managed/native signatures did not match anyway)
Add new NSUrlCredential.cs file in build
Add missing [NullAllowed] attributes in NSLayoutConstraint [#10984]
Fix monomac to work with async.
NSGraphicsContext needs to be marked with [ThreadSafe] since CurrentContext returns a context that is valid for the current thread. Also register NSPrintPreviewGraphicsContext or it won't be possible/easy to use the context for the print-preview. ref: desk #30220
Do not reset MCS if one was already supplied
Add missing [NullAllowed] attribute to NSWindow ordering methods
MonoTouch.Constants and MonoTouch.MonoTouchException are not namespaces (and can conflict with types when compiling using newer compilers)
Preferences Window Sample

Sample project to demonstrate how to implement preferences window as
seen in many Mac applications.
Update README.md