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.
Instead, I'd prefer you add near the top:

    #if __MAC_OS_X_VERSION_MAX_ALLOWED < 101000 // Apple recommends not to use the constants
    NS_EXTERN NSString *const NSURLQuarantinePropertiesKey;
    #endif

reverse the if-else condition, and change the ifdef to a `#if __MAC_OS_X_VERSION_MIN_REQUIRED < 101000`. Then all SDKs compile in the same behavior. This also makes future diffs cleaner since when support for deployment to a particular version is removed, you only need remove lines rather than restructure the code.

See https://github.com/sparkle-project/Sparkle/blob/master/Sparkle/SUHost.m#L15 and https://github.com/sparkle-project/Sparkle/blob/master/Sparkle/SUHost.m#L259 for a similar example to what I'm talking about.