Update ZpClient.js

- Read `device_description.xml` on `init()`;
- Retrieve zone information from ZoneGroupState on `init()`, unless already passed to `constructor`;
- Provide properties for most used attributes from `device_description.xml` and ZoneGroupState;
- Add SOAP actions used by homebridge-zp;
- Rename `deviceDescription()` to `get()` and cache responses.  Also support getting the icons.