Quarantine removal

The following changes expand the removal of the quarantine metadata information.
On 10.10 and Xcode 6, it uses the new `NSURLQuarantinePropertiesKey` key, setting the value to `[NSNull null]` as recommended by the Apple docs.
    #import <Foundation/Foundation.h>

    #if __MAC_OS_X_VERSION_MAX_ALLOWED < 101000
    extern NSString *const NSURLQuarantinePropertiesKey WEAK_IMPORT_ATTRIBUTE;
    #endif

    int main()
    {
        if (&NSURLQuarantinePropertiesKey)
            NSLog(@"%@", NSURLQuarantinePropertiesKey);
        else
            NSLog(@"not found");
        return 0;
    }

Compile with `clang test.m -framework Foundation -mmacosx-version-min=10.7 -Wl,-U,_NSURLQuarantinePropertiesKey` - outputs the value of the key on 10.10, outputs "not found" on 10.9. This has nothing to do with weak linking the Foundation framework as a whole.

This is both backwards and forwards compatible and results in the same behavior on both OS X 10.9 and 10.10 regardless of the SDK it was built with (which is a valuable result for little effort - easier than dlopen/dlsym too).