Comment RegEx in Flat Rate Shipping

Regular expressions can get quite long and confusing. This makes them hard to debug (as I was doing today). Other languages like Perl have commenting built into their [regular expression functions](http://perldoc.perl.org/perlre.html) (look at the x flag) to handle this. PHP doesn't have that fanciness - I simply broke the pattern into a multiline string and commented on the relevant parts. It will make any future debugging, or editing much easier.