Improvements to packer/packMJfile

Thanks for merging the previous pull request, here's the remaining part of our delta:

* Do `$dstname =~ s/$cwd/\/MathJax/;` to avoid paths like `/tmp/buildd/mathjax-2.0/jax/output/HTML-CSS/jax.js` in headers of compiled files. I would personally prefer simply `$dstname =~ s/$cwd\///;`, but this change makes the paths equal to those in MathJax.git.

* Try to find YUI compressor in `/usr/share/yui-compressor/yui-compressor.jar` (as found in [our package](http://packages.debian.org/sid/all/yui-compressor/filelist)), and only then fall back to the local path.