add gold standard flag to classification

If the user is an 'expert' on the project then add a gold standard flag to the classification, closes #237
Might as well go for the flexibility of allowing experts to be assigned at any time. I just think there is some danger is allowing experts to chosen at any time - just because someone has done lots of classifications does not make them an expert. I think people should be forced before hand to determine who the experts are. Definitely something to get Ali's opinion on.   @aliburchard