Spike event stream urls

### References
* https://www.pivotaltracker.com/story/show/82983252

This is a spike that is open for review.  Any feedback on this approach is welcome.

### General Implementation
* when a resource changes, calculate the payload that ember store needs and store this payload in a redis cache with a unique guid
* send down the guid of this payload to the user via the event stream
* client makes an HTTP request for this payload and it is used to update the ember store

### Implementation on destroys
* when a resource is destroyed, send the payload itself down the `system` event stream channel to all users
* resource is destroyed from the ember store immediately without making an HTTP request to the server

### Thoughts
* this is a pretty straight forward implementation that leverages much of the event stream work already in place
* the implementation for destroys could have followed the same pattern as creates / updates, but it seemed needless to have all clients making http requests against the server only to check if a resource should be destroyed from the store
* storing the precalculated payloads in redis makes the responses very fast
* my only concern is any future performance implications of having many clients hitting the server to obtain payloads