spin 1.71

I managed to trigger the spin Perl warnings about undefined variables caused by a missing argument to a command today, and took a moment to fix it. Missing arguments are now set to the empty string rather than an undefined value, and \size now produces a good error message about an empty file argument. There are probably other commands that will need a similar fix, but I'll get those as they show up.

You can get the current version of spin from my web tools distribution page.