Use singular name in taxonomy context labels

As far as I can think, there's no need to have a pluralised category name anywhere. It makes more sense to have use the singular name as the context.

Here's a before and after shot (plural before, singular after):

![screen shot 2014-09-18 at 09 41 14](https://cloud.githubusercontent.com/assets/1097667/4313151/6de7b5ca-3ec4-11e4-92a9-75ea4734c10f.png)

@fjarrett Please review. Also, please try to think of a use case where a plural context would be required. I can't.