Added support for Rain Gauge

Added mapping for Rain measurement in SENSOR_PROPERTIES and updated
usage documentation.

Running samples/printAllLastData outputs:
```
Rain Gauge
                     rf_status : 60
                    battery_vp : 6402
                          When : 06:39:45
                          Rain : 0
Indoor
                         Noise : 34
                   Temperature : 21.2
                          When : 06:39:53
                      Humidity : 31
                      Pressure : 1035
                           Co2 : 542
Indoor 2
                           Co2 : 468
                     rf_status : 63
                   Temperature : 23
                          When : 06:39:38
                      Humidity : 27
                    battery_vp : 5882
Outdoor
                     rf_status : 59
                    battery_vp : 6132
                          When : 06:39:51
                   Temperature : 0.3
                      Humidity : 61
```