Parsing of "Buddylist Overlay"

In `_parseMessage`, you will recieve a lot of data with the type `buddylist_overlay`. Sample `msg`:
```json
{
"seq": 50,
"t": "msg",
"u": 1234567890,
"ms": [
{
"type": "buddylist_overlay",
"overlay": {
"1234567890": {
"a": 2,
"c": 4170,
"s": "push",
"vc": 74,
"la": 1518985361
}
}
}
]
}
```

The `'a'` (activeness?) value appears in two (and maybe more?) states: `0` and `2`, where `0` means offline and `2` means online.
The `'la'` (last active?) value indicates the timestamp

This can be used to know when users become online or when they go offline (in real time, compared to `onChatTimestamp`, which only recieves these updates once in a while).

If anybody can figure out what the `c`, `s` or `vc` values could mean, please let me know (!), otherwise I'll probably make another `onX` function at some point.