Add persistent storage with redis (bug 959248)

https://bugzilla.mozilla.org/show_bug.cgi?id=959248

* replaces `save` with `then-redis` (you have to launch an instance of Redis to try)
* replaces `Q` with `when` to handle promises
* turns some queries' logic into map/reduce given that Redis is a key-value store
* turns all views into promise-based logic (particularly sellers)
* gets rid of all `_id` introduced by `save`, now each object has a `uuid` property
* adds consistency on `function ()` vs. `function()` (no more extra space)

Still missing:

* [x] adding `then-redis` and `when` modules (waiting for readability of the PR)
* [x] adding a mock for Redis testing (declared a new bug: 965207)