added gw codes

I added the state coding of [Gleditsch and Ward](http://privatewww.essex.ac.uk/~ksg/statelist.html). For the most part it is the same as COW numeric codes. Some states that are considered states by COW are not considered states by GW, and vice-versa. I informally tested the regex for the additions (it broke some of the states already in the data). I haven't figured out how to match the Korea that existed prior to the Japanese invasion though.