Optimize clustalw report

This is an optimization for Bio::ClustalW::Report#do_parse.

I've written a benchmark, and the change in execution time can be seen at https://gist.github.com/agrimm/6593082 . On MRI 2.0, performance is approximately 70% faster.

This optimization was done without any knowledge of what clustalw output ought to look like. It's possible that someone familiar with clustalw output could create a better optimization.