This introduces a generic way to register certain events emitted by
the js-sdk as those that should be propagated through as dispatched
actions.
This allows the store to treat the js-sdk as the "Server" in the
Flux data flow model. It also allows for stores to not be aware
specifically of the matrix client if they are only reading from it.
These can be used to dispatch actions immediately, or after some asynchronous
work has been done. Also, create GroupActions.fetchJoinedGroups as an example.
The concept of async action creators can be used in the following cases:
- stores or views that do async work, dispatching based on the results
- actions that have complicated payloads, would make more sense as functions
with documentation that dispatch created actions.