Phar fixes

Fix issues with path handling when Phing is packaged as a phar.

I've been unable to get Phing to run from a phar archive, which I've compiled. I can get the packages from the website to work with a certain amount of tweaking but I'd like to have a standalone copy of Phing and dependencies (like Archive_Tar).

There doesn't appear to be a branch for the 2.4 series, so I've made the PR against master but when is this branch indented to be stable?