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.
What do you mean by weak link? As far as the deployment target is concerned, the symbol is set to be weak-linked. If you want to weak-link the library the symbol is in: I don't think linking the whole Foundation framework weakly would be ideal. If you want to load the symbol via `NSBundle` or `dladdr`, the current method is cleaner.

And yes, the ampersand sign is important: it makes it so older versions of OS X crash when doing the check.

Sent from my iPhone

> On Jul 19, 2014, at 1:11 PM, "Jake Petroules" <notifications@github.com> wrote:
> 
> In Sparkle/SUPlainInstallerInternals.m:
> 
> > -    // Only recurse if it's actually a directory.  Don't recurse into a
> > -    // root-level symbolic link.
> > -    NSDictionary *rootAttributes = [[NSFileManager defaultManager] attributesOfItemAtPath:root error:nil];
> > -    NSString *rootType = rootAttributes[NSFileType];
> > -
> > -    if (rootType == NSFileTypeDirectory) {
> > -        // The NSDirectoryEnumerator will avoid recursing into any contained
> > -        // symbolic links, so no further type checks are needed.
> > -        NSDirectoryEnumerator *directoryEnumerator = [[NSFileManager defaultManager] enumeratorAtPath:root];
> > -        NSString *file = nil;
> > -        while ((file = [directoryEnumerator nextObject])) {
> > -            [self removeXAttr:quarantineAttribute
> > -                     fromFile:[root stringByAppendingPathComponent:file]
> > -                      options:removeXAttrOptions];
> > +    NSFileManager *manager = [NSFileManager defaultManager];
> > +#if __MAC_OS_X_VERSION_MAX_ALLOWED > __MAC_10_9
> That's why you weak link it.
> 
> —
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