Filter items in a reactive way

[bryant1410]

Right now, the filter is not applied to an Observable. Maybe a better approach is that entities are directly got as Observables, and then a filter can be applied. So this should return an Observable instead: https://github.com/Jearil/SimpleNoSQL/blob/master/SimpleNoSQL/src/main/java/com/colintmiller/simplenosql/threading/DataDispatcher.java#L140