KIT2FIFF GUI: manually set stim channels

Someone in our lab using a new stimulus presentation software/setup that sends triggers to different channels, this PR allows setting the stim channels manually. Would be nice to get in for 0.8.
![screen shot 2014-06-28 at 12 16 38 am](https://cloud.githubusercontent.com/assets/145771/3419782/0cf12a32-fe7b-11e3-8223-f825dddfb82e.png)