Add Observer Sets

@mikewalch would like to make it easier for multiple developers to run workers and observers. We were discussing how to do this. One possibility we came up with is having observers sets. Each observer set would have a list of Observers. A developer could kick off a yarn application that runs workers and one or more observers sets. The yarn application would only execute the observers in its sets. The yarn application could reserve the observer set to keep other yarn applications from running those observers.