make stores to functors like actions

This PR turns stores into functors, much like actions. I.e. doing `store.triggerAsync(data)` is now the same as doing `store(data)`.

One option for cleaning up the docs in the future could be to start with the point that at the heart of Reflux are two API:s, listeners and listenables. Actions implement the latter, React components the former (through the mixin), and stores both. Having actions and stores behave in the same way (with regards to publishing) would help get that across.