The improvements we want to achieve:
* Stability - No more corrupt / incomplete updates by ensuring atomic move operations
* Efficiency - Faster updates because of reduction of file copies that need to be done
* Separation from Host and Helper - No usage of SUFileManager in the host application
* Cleanness - Minimized deprecated APIs (no FSPathMakeRef, FSGetCatalogInfo, FSFindFolder, etc), use modern Cocoa APIs when possible, robust error handling, strong documentation, easier to read code.

Many other small notes:
* SUFileManager does not have class methods. Methods across an instance share an AuthorizationRef, instead of having to authorize every time a privileged operation needs to be done
* The odd logic of `while (authStat == errAuthorizationDenied)` was removed
* Instead of checking if we have write access, we try to invoke a normal operation first. If that doesn't work, with the right error returned back, we try authorizing, which is what Apple recommends doing
* The trash is not used as a temporary location anymore. Rather we now request for a temporary directory to be created. This is more likely to succeed too (eg: on a drive that has no trash folder)
* The old method for releasing the quarantine did not work until I used the "com.apple.quarantine" key instead of the LS constant that was being used.
* For the old method for releasing the quarantine, we now check if a file has the quarantine xattr bit set before removing it. Just like how the new method works.
* Quarantines can now be removed with authorization
* The result of calling temporaryNameForPath: was never actually used in the old code
* -[NSString fileSystemRepresentation] which can throw an exception is not used. Only getFileSystemRepresentation:maxLength: is used and the error on it is always checked
* The desperate/hopeless check for if the code signature for the update is valid after the installation is complete is removed

Potential issues that still needs to be looked in to:
* Currently the method to move a file to the trash only works on 10.8+
* Appending of version number / picking destination name of app moved to trash

Since it is possible for the user to see these strings, we should not make them seem as 'gibberish' or unfriendly. More detailed error information is logged to the console.

I don't think this is actually needed, but better be safe than sorry.

By not taking account the rare case, we simplify the code by having
less duplication. The tradeoff is that the rare case will be slightly
less efficient.