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.
The problem is, if we set it so that it compiles line 569 on Xcode 5, the linker will fail with an error because it can't find symbol NSURLQuarantinePropertiesKey in any library. The key was added in 10.10, according to the documentation. 

However, the check on the aforementioned line (when built against the 10.10 SDK) will see if the symbol is available and, if it is, will execute the 10.10-specific code. Yes, I have done this before in my own project. Yes, it does work.