Object Key in data inspector

[tmarzec]

_objectKey property can now be seen in objects when exploring them with the data inspector. Ideally, this property would be hidden. Currently, due to the complexity of converting objects upon receiving them from the device (also see #84).

This could be solved in a number of ways. Firstly, this property could be defined as nonenumerable (after receiving from the device, since otherwise it would be ignored by serialization). Secondly, the data inspector could be changed, so that it uses schema.properties. Second approach makes more sense if we would need to store more fields in objects.