Implement POSIX.1-1988 UStar directly instead of calling the tar binary

The Tar format is really really simple, so I implemented it in PHP instead of having to call the tar executable. This has many, many advantages :

* Support for all platforms (Windows, Linux, etc)
* Will work fine on hosts where `safe_mode`, `open_basedir` or function blockades are active
* The UStar format isn't likely to change
* Not much can go wrong
* It's a lot faster and involves far less buffering