Added WaterLevel (depth) for SmartMi Humidifier - model: zhimi.humidifier.ca1

[leoneleone]

@aholstenson
The new Xiaomi Humidifier (zhimi.humidifier.ca1) has a Water Level sensor, the capability is named 'depth' according to the Mi API

I've added WaterLevel using RelativeHumidity as a template.
Please let me know if this looks alright.

I've also submitted a pull request for abstract-things
thingbound/abstract-things#3

I would like to test my changes, but I'm running into miio handshake timeout errors.

ERROR:  { Error: Could not connect to device, handshake timeout
    at Timeout.handshakeTimeout.setTimeout [as _onTimeout] (/usr/local/lib/node_modules/miio/lib/network.js:427:17)
    at ontimeout (timers.js:460:11)
    at tryOnTimeout (timers.js:298:5)
    at Timer.listOnTimeout (timers.js:261:5) code: 'timeout', device: null }