Perl 5.8.8 compat

So, an effort to port LaTeXML to be backwards compatible with Perl 5.8.8.

We should discuss and finalize what to do with the "nonempty" subroutine I have defined in each of Scan.pm, CrossRef.pm and TeX.pool...

Also, File::Path decided to rename ```rmtree``` to ```remove_tree``` roughly at version 2.0, so I bumped the required version for that module to 2.07, since I am using ```remove_tree``` for cleaning up after the archive sandboxes. **EDIT:** There was a better solution, it seems ```rmtree``` is still supported, so I switched to that and removed the version requirement.

Similarly, Getopt::Long has changed its internal datastructures after [version 2.37](https://metacpan.org/changes/distribution/Getopt-Long#L64), so I recorded that as well. 2.37 of Getopt was released in 2007, fyi.

Next, we have [named backreferences](http://perldoc.perl.org/perlretut.html#Named-backreferences), used in xcolor.sty.ltxml. That code is really elegant and thinking of refactoring it for something worse/older makes me sad... I haven't done that yet.

I can report that LaTeXML's ```make test``` passes all tests except the xcolor-including ones, with the suggestions in this pull request.