Add streaming API documentation

- streaming: add sub/unsub OP codes, changing from the old behavior of
    just giving a channels array.
 - streaming: properly check payloads against their data and send
    proper errors.

 - manager: add unsubscribe()

docs/streaming.md

@@ -0,0 +1,82 @@
+# Streaming API
+
+Elstat's Streaming API enables clients to receive realtime
+updates from the server.
+
+Clients can subscribe just to *certain* channels in the case
+they don't want to have a lot of overhead, or they can just
+not use the Streaming API at all. It is optional.
+
+
+## Common payload format
+
+Payloads can not exceed 256 bytes in size.
+Payloads are JSON encoded.
+
+```javascript
+{
+    "op": ANY_INTEGER
+}
+```
+
+Extra fields are on each OP's description, instead of being in
+some `d` field (like Discord's Gateway API).
+
+
+## Error codes
+
+| Error code | Name |
+|---:|:---|
+|4200|`INVALID_PAYLOAD`|
+|4420|`TOO_MUCH`|
+
+
+## OP Codes
+
+| OP Int | OP Name | Sent/Received by client |
+|--:|:--|:--:|
+|-1|`UNSUBSCRIBE`|Sent|
+|0|`SUBSCRIBE`|Sent|
+|1|`SUBSCRIBED`|Received|
+|2|`UNSUBSCRIBED`|Received|
+|3|`DATA`|Received|
+
+
+## OP Payloads
+
+ - `SUBSCRIBE`
+    - field: `channels` (`list[str]`), array of channels you want to subscribe to.
+    - reply: `SUBSCRIBED` payload.
+ - `UNSUBSCRIBE`
+    - field: `channels` (`list[str]`), array of channels you want to **un**subscribe from.
+    - reply: `UNSUBSCRIBED` payload.
+ - `SUBSCRIBED`
+    - field: `channels` (`list[str]`), array of channels you *succesfully* subscribed to.
+ - `UNSUBSCRIBED`
+    - field: `channels` (`list[str]`), array of channels you *succesfully* **un**subscribed from.
+ - `DATA`
+    - field: `c` (`str`), the channel the data is coming from.
+    - data: `d` (`list[any]`), the data coming in from the channel.
+      - The first element of `d` is an integer, encoding an UNIX timestamp with millisecond precision.
+      - The second element of `d` is described on *`Channel types`*
+
+
+## Channel names
+
+Channels are what clients subscribe to receive data about that channel.
+Channels are specified as `<type>:<name>`
+
+e.g `status:elixire` and `graph:elixire`
+
+### Channel types
+ - `status` channel
+    - Returns a boolean, representing the status of the service
+ - `latency` channel
+    - Returns an integer, representing the latency of the service, in *milliseconds*.
+
+
+## Connection logic
+
+ - Connect a websocket to `/api/streaming`.
+ - Send a `SUBSCRIBE` payload, receive `SUBSCRIBED` back
+ - Listen to `DATA` payloads and update local state as needed.