AM2320 driver does not handle negative temperatures correctly

[jerryneedell]

The AM2320 sends a 16 bit temperature value with the MSB indicating sighn and the low 15 bits indicating magnitude. The driver as written does not handle negative values correctly.
I put the sensor in a freezer:

Temperature:  0.2
Humidity:  14.5
Temperature:  0.1
Humidity:  14.6
Temperature:  0.1
Humidity:  14.6
Temperature:  0.0
Humidity:  14.7
Temperature:  3276.9
Humidity:  14.8
Temperature:  3276.9
Humidity:  14.8
Temperature:  3277.0

A simple fix is to change

def temperature(self):
        """The measured temperature in celsius."""
        temperature = struct.unpack(">H", self._read_register(AM2320_REG_TEMP_H, 2))[0]
        return temperature/10.0

to

def temperature(self):
        """The measured temperature in celsius."""
        temperature = struct.unpack(">H", self._read_register(AM2320_REG_TEMP_H, 2))[0]
        if temperature >= 32768:
            temperature = 32768 - temperature
        return temperature/10.0

I tried this in the freezer

Temperature:  0.1
Humidity:  17.6
Temperature:  0.1
Humidity:  17.7
Temperature:  0.0
Humidity:  17.7
Temperature:  -0.1
Humidity:  17.8
Temperature:  -0.1
Humidity:  17.9
Temperature:  -0.2
Humidity:  17.9
Temperature:  -0.3
Humidity:  18.0
Temperature:  -0.3
Humidity:  18.0
Temperature:  -0.4
Humidity:  18.1
Temperature:  -0.4

If you concur with this change, I can put in a PR.

[dhalbert]

This is a nice simple fix, easier than extracting the sign bit. Thanks! A PR would be welcome!

[dhalbert]

Fixed by #2